You have a fault with the caliper; my understanding is that Mazda uses a Lockheed-Girling design also licenced by VW and Rover, amongst others. They too suffer the same hand brake-self adjustment issues if poorly maintained.
Cue someone to chime in with “my calipers are fine”, but not quite. On my 1.6, I fitted recon calipers after 100k miles, as they became marginal for handbrake efficiency. In 11 years of MX5 ownership (daily drivers and hillclimbers), these are the only calipers I needed to change. I am not aware of anyone who has had to change calipers every 2 years; whoever told you that is talking porkies. My other 1.6, a 93 M2, had rears that gave no problems. Present car, 96 1.8, now on 200k kms, 110k of that down to me. Can’t remember the last time I had to adjust the rear brakes; last time I changed the pads/discs which was some time ago. For some reason, 1.8 brakes don’t seem to have the same issues as the 1.6, which is strange, as the caliper ought to be the same.
I can understand your points why you are interested in the RX7 FC brakes; not sure they are more easily available than the Sport brake kit (a large proportion of Mk2.5s are Sports), which uses a completely different caliper than other MX5s. Not that many RX7s were sold in the UK.
I still think it will be far more efficient getting your present braking system into the state Mazda intended, before passing judgement on it. But its your time/money. You asked for advice, opinions, you got it. The UK MX5 community is relatively small; you might get better answers over on miataforum, as suggested, which is far bigger, and may actually have someone who has done what you want to do.
A Thai owner fitted RX7 brakes, but he went for fitting the hub as well, and hence had to change to 5-stud wheels.
No doubt the RX7 brakes, if not bodged, will give a stronger bite, but I’m not sure a 1987 era caliper will have a more reliable handbrake (MX5 calipers do not need regular readjustment) as reportedly these also have a bit of a weak handbrake.
Another MX5 FD fitment:
Malcolm Gray-Stephens I think is the authority here on Lockheed brakes; was on the old forum, not sure he’s made it over yet. You can still PM him on the old forum website.
His homeplage of random technical snippets:
http://www.quithel.co.uk/zz_Brakes.htm
I’m just smirking at the moment :p. A mate has managed to take apart the siezed one so im part of the way to fixing them. Just need to refurb them completly now and take it form there 
I think it needs mentioning that a well maintained mx5 brake setup is an extremely good braking system.
Where people go wrong is when they jump into a 5 after driving a modern over servo’d barge.
The ideal is a brake that offers good feel and modulation without locking up the brake easily.
That’s pretty much every mx5 brakes i’ve driven.
My car is putting out approx 240bhp, it’s a Auto so no engine braking and i track and compete in it as well as use it for my daily transport.
My brakes are 100% stock and even after 20min track sessions they still operate well.
My case is not really the odd one out, there are thousands of mx5’s competing around the globe happily using 100% stock brakes.
Another thing worth baring in mind is that anything past locking up the wheels is totally and utterly wasted.
The key is to balancing the braking system so it needs a fair prob to lock up the wheels, exactly what the 5 has.
My advice is bleed out your braking system and try some different pads.
The Axxis ultimates are the best i’ve tried yet.
Really the stock brakes are fine.
Cheers
Mark
I just had a thought.
Have the “Big disc” MX-5’s got a different spare wheel to get over the larger setup? Without going out to the car I don’t even know what size the space saver is ?
I’m going to guess most of them don’t bother with a spare; its excess weight after all.
Ohhhhh, Ar$e fudge !
Last night coming back from the family Christmas prezy runs, I noticed my drivers side front started rubbing slightly. I thought “OK, Briliant, Another sticky calliper, Only 4 more miles till home, I better check it out tomorrow…”
By the time I got home my car had that lovely aroma of flambeed brake pad and a judder that tells me my disc’s now the shape of a pringle !
I’ve got a spare caliper to fit and I just hope there’s enough meat left on the disc so I can just skim it flat for the time being :(
I’ve ordered the 270mm discs and have some RX7 front calipers here so I don’t want to buy any more standard discs, I just hope I can get away with a light slim for now.
MERRY FRIGIN’ CHRISTMAS ! (Baa, humbug) 
A little update on here, After a lot of head scratching I’ve realised why RX7 callipers aren’t on every tuned MX5 out there.You would need to use the 270mm disc’s and 20mm spacers because There isn’t enough clearance between he callipers and the radius on the sides of most wheel’s mounting face… But !
I’ve been looking into this on and off for the last 7 months and I now think I might have a solution.
The 294mm x 22mm BMW Mini John Cooper Works disc’s would clear the wheel mounting area and should just fit behind my 16" Hockenheim’s with the callipers and without using wheel spacers.
With these disc’s I’d get even more room to make the mounting plates and the callipers would have SOOOOOO much leverage over the hubs that I couldn’t see me wanting to ever upgrade (The front’s [:D]) again. They’d also visually FILL the space behind the 16" wheels nicely.
There are a few problems with this idea however…
1 - The 294mm disc’s will be starting to get on to the “Ridiculously heavy” end of the scale
2 - Even aftermarket versions of the JCW disc’s are £150+, So that knackers up the “Cheap upgrade” idea.
And 3 - BMW’s just slapped a recall on the John Cooper Works brake’s and don’t go into much detail as to what the issue is !!!
As a concept, I think it might work and If I can find some disc’s cheaply I’ll try it out… But in practise I think the horse I’m beating could be drawing it’s last few breaths [:^)].
Just for you to drool over Scotty, here’s a pic of a big brake setup on a Mk3, I reckon this one will stop ok—
Have to say I’m in the school of “theres not much wrong with standard”. Our 94 mk1 brakes have had all the callipers replaced about 3 years ago and other than new pads have been fine. My 99 mk2 is totally standard (same set up without ABS) and its easy to lock the wheels if you want to, and as has been said any more is pointless. What would be welcolmed is some stainless pistons and some decent rear caliper components that wont seize!!!
I think maintenance is the key, stay on top of that and the std brakes will be ample.
G’day Dave
I totally agree, my brakes are the standard 1.6 discs and Mazda pads, they stop me fine, and I have never experienced any fad, even in the French Alps a few years ago—
There may be a problem with you BMW MINI idea. When I put the twin pots on mine I had to use spacers so they didn’t hit the wheel. Had I used a different disc with the friction face set further in I wouldn’t have needed the spacers, but the discs would have fouled on the hub. When I put 1.8 front discs on the back I had to remove the dust shields and there was only a few mm of clearance between the disc and the hub.
I have a 98 RS import 1.8 6 spd, excuse my apparent ignorance but does this mean that it has the bigger discs?
Cause mine which had new pads/discs when i bought it a few stops extremely well with no sign of fading and seems well balanced, or am I just lucky[:D]
Your car has exactly the same brakes as a Mk1 1.8, which means bigger discs, slightly bigger pads, and repositioned calipers, compared to the earlier 1.6. After the 2001 facelift, Mazda introduced a Sport option which had even bigger discs, but I think a different caliper to the earlier cars (making it more of an expensive upgrade.
Any MX5, if in good condition, will have well balanced braking, and lack of fade on the road.
I don’t think you are lucky, its just as is should be IMHO. I recently had to change front pads on my MK1 1.8 and found the replacements were too big for the discs. When I looked into it I found out that a well known supplier in Croydon had replaced my previous worn discs and pads with those of a 1.6! (calipers were in the same position). However they performed just as well as the proper fitment with good no fade braking.
So the standard fitment must be well capable for road use if fitted and maintained correctly. (I rectified the problem with new discs and pads)
BB
PS on another note - why did someone say ther is no engine braking on an auto? Mine slows down very quickly when I lift off the gas.
Time to admit defeat. [:(]
A few weeks ago I was offered a set of callipers, Hangers and pads from a MK2.5 sport… So I bit the bullet and got them.
To be honest I was getting sick of looking around in scrap yards to see what other rear callipers could be adapted (Then going home, Looking up the specs and discovering that the piston size is way out [:S]).
So all I’m going do now is cleaning them up, Painting them and (If I can find the kits) replacing all the seals.
I’m a bit annoyed that after all the research and hunting I wind up following a more conventional path to larger brakes, But I suppose If the kit works well, Then it’s the destination rather than how I got to it thats important.
Scotty B.
PS, Anyone want to buy some RX7 FD four pots ? [:O]
I could make those brackets for the fc rx7 brake conversion for alot less than that company is charging and you wont have import charges, If someone could draw me an exact CAD drawing of them or even on paper would be much appreciated… help me to help you
Back in 2009 when this thread was live I had some spare FC RX7 4-pots laying on the workshop floor. I could have CADed them up in glorious 3D and then used the data from my AP twin pot conversion to design the bracket.
Sadly I don’t have the calipers or the data any more.
ahh thats a shame man didnt realise this thread was soo old 
maybe someone else will be able to help desing some…