First off, anyone using used suspension does so at their own risk.
Can’t tell you if its “junk” or not, but Koni have a pretty good reputation. But, you will get a better ride on the Mk2 Bilsteins than on the Mk1 Konis, because of the increased Mk2 shock travel. Lowered Mk1s suffer from riding around on the bumpstops.
Controversially, I’m going to say for 95% of people, none. There are those looking for a certain ride height, for aesthetic reasons. The adjustable seats allow this, though really, they ought to corner weight as well. Bounce/Compression knobs; the only time I’ve bothered to reset these on my Performance5 Puredrive Protecs was when some cold weather caused the shocks to become overly stiff. I’ve never bothered to change the spring perch settings from the setting recommended by Performance5.
If you fit the Mazda Billies, even the Mk2 versions, chances are you could get away from not being put into a modified road going class, which if you are looking for position, can be pretty depressing in hillclimbs/sprints (typically, you’d be up against barely road legal stipped out Minis and Novas arriving on trailers…). I’m not going to do your price negotiating for you, but lets just say, you could try a bit harder on those Billies.
GAZ, like Protec, are a tiny tiny company, compared to Koni et al. Which means, like Protec, you’ll get a shock with a lot of bespoke features. But you will also get more than your fair share of issues to do with low production numbers, inadequate testing, inadequate spares support and so forth. Got a problem with the shock? Unlike a Koni etc, which will be just swapped from stock, in all probability, you will have to put up with the car being on bricks, while they fix the shock for you. I wish I could sing the praises of a British company; but these GAZ and Protech products are really aimed at a different market from Koni etc. Packed with features, for sure, and you can probably set up your MX5 beautifully with them, but don’t expect to use them flogging up and down the motorway.
GAZ and Protec have their share of problems. I didn’t go for GAZ, because I wasn’t impressed with their zinc plating for corrosion protection, and the less than adequate design, which lead me to believe their original testing had been done on 1 car (eg. wrong nuts sent out with cars, leading to rattling shocks)… I took a plunge with the Protec kit, because, on the face of it, it had excellent corrosion resistance, and seemed to have undergone quite a bit of development, plus Protec have an excellent reputation with the TVR crowd. 2 years on; the shocks are still very shiney, with nice clean threads. Unfortunately, the adjusters are seizing up, due to electrolytic corrosion (in my opinion; Protec disagree, and claim these problems are only happening on MX5s, not on TVRs). I’ve had 1 seal failure, and a shock returned for adjuster replacement. The turnaround is about as quick as I expect.
Now Protec(h) are apparently coming out with an improved shock design for the MX5, to go with the 2.5" springs. New knobs and extensive salt testing seem to have “cured” the issues (well, apparently 20,000 miles of simulated salt testing resulted in a shock with a still functioning knob). Annoyingly, the new knobs cannot be retrofitted to earlier designs (here’s an opportunity for anyone out there; come up with a rubber knob cover for Protechs (and Gazs); Protec will bite your hand off)… I dare say there are similar gradual improvements with GAZ shocks, and someone will be here to sing their praises.
Fact remains, none of these small manufacturers can get the same kind of componant testing as Koni, Bilstein, Showa, Tokico and KYB, all of whom are OE suppliers. Most aftermarket Japanese brands actually use versions of mainstream shocks, to maintain reliability and a modicum of QC. Chinese shocks tend to be copies of these mainstream shocks, which means you won’t get failure due to poor design, but more likely to poor workmanship (shock eye welds breaking off) and/or poor material (changes in seal material specs). I’ll put in a mention for Tein as well, which I think don’t use KYB etc bits. They are quite expensive (seems to be more a case of trading in on a name than anything else), but they don’t seem to hold up to use in the UK well (painted steel bodys, seem to corrode quite easily, like KYBs. On the other hand, a set of steel Bilsteins I had were still in very good condition after 7 years of UK winters). The Korean and taiwanese shocks in circulation are a bit of a unknown quantity, but I suspect most of the users of these aren’t all that concerned with ultimate life when used everyday.
Not sure thats done anything to make it clearer for you; I suggest you have a good look through the many many user reviews over on Miataforum, to get a balanced view.
