Does a stainless steel back box, KN or equivalent panel filter and a brake upgrade to a Mk1 have to be declared to your insurer as they are non-standard?
As far as I can see none have any real impact on performance. Indeed better brakes are a good upgrade as the stock brakes are not great in every day driving,
setting aside track days (which I won’t be doing).
Probably depends on the insurer, some aren’t interested in anything that doesn’t change the power output, some want to know anything. Only way to be sure is ask the company you are insured with.
All of those items are going to be more expensive to replace than the standard items in the case of an accident and so will cost the insurance company more and so yes, they probably will want to know.
You declare them all. Its not about whether x modification adds z hp, its about drivers with x modification presenting a different risk profile compared to those without. Some will decide not to charge an additional premoum, others may.
Standard brakes are perfectly adequate when maintained correctly.
As others have said, ask your insurer, and if you change insurer at renewal time, remember to state the changes.
Not saying it has ever happened, but if you knowingly mislead the insurer by saying ‘No’ to the question on 'Has the car been modified from standard specification? ’ Then in theory you are liable to have your insurance voided. As I say I don’t know of examples where this has happened, but if say you had an accident, and accident investigators found that a disc rotor cracking had affected/contributed to the accident, could be a tricky conversation if you were running an eBay special brake kit they didn’t know about.
I have an LSD and suspension upgrades on my BMW, at a £3k+ cost, and this added nothing to my policy cost. Upgraded suspension on my MX5 added about £50. Go figure…
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Thanks for your replies. It seems to me therefore that it is better to be safe than sorry and declare any non-mx5 supplied parts.
I told my insurers Admiral that I’d had the car lowered, fitted a new back box, fitted a different interior (seats/door cards) although just a colour change really and updated the alloys with the later model Mazda alloys (same size) fitted an aftermarket head unit and fitted various bits of tacky chrome everywhere. 
They were only interested in the lowering of the car (added to the premium) but recorded the back box, alloys and head unit change for their records. The bling so long as it wasn’t body kit they weren’t overly bothered.
Aviva have a list of modifications you can check off and as far as I can remember weren’t interested in anything else.
I told my insurance about lowering spring and s/s back box they recordered on my policy but no extra charge…
Thanks for all your recommendations and advice.
Are there any standard MK1’s?
How many can say they went to a Mazda dealer and had them fit a replacement exhaust or air filter?
The car is a minimum of 22 years old, I don’t expect insurance assessors are too worried if the exhaust is made of mild steel or not as they are going to write it off for as much as a blown bulb.
I asked Admiral for an “Agreed Value” policy on my Eunos which has been tweaked but the only performance mod was the timing change and a cat back Cobalt exhaust. It has a long list of mods (including suspension) and I had to send photos and a detailed list to their engineer, who approved all of them.
The car is on a car/home multi policy and I pay less than £100 for fully comp but I am retired and have no at-fault accidents in 50 years of motorcycle and car driving. Car insurance has 5,000 miles annual mileage as “typical” but in real terms is probably nearer 2,000 annually - 2019-2020 will almost certainly be less than 1,000 which is a crying shame 
If you do or do not have mods, and do not have an agreed value policy, then the car will almost certainly be written off for even small damage and “book value” used in settlement. Car then becomes a Cat D(now called N) or Cat C(now called S) and is marked on the records at DLVA.
“What is a Cat C (now S) car? Cat C (now S) cars are what insurers call a repairable total loss: the cost of repairing them is greater than the car’s value. That’s unlike a Cat D (now N) car, where the cost of repairing the car is less than the vehicle’s pre-accident value.”
Thanks for your email, duly noted!