why are my ears burning?
I actually have two 1st gen(88-92)MX6s. The daily driver/GS pseudo-racecar and a 91 which is the DSP car. I only have knowledge of the 2nd gen car in a removed sense since I have never owned one.
That said, heres my thoughts....
Anything mazda is not a bad thing. Why? Mazdaspeed motorsports. A treasure trove of parts and services for dirt cheap. This is perhaps the best thing about competing in a mazda.
Track: The car is very well balanced from the rides that I have swiped from others. Its relatively light and the chassis is a pretty good base. The mighty-mite V6 puts out a decent amount of power (prior to the Turbo MZRs, Mazdas power output could always be descried as "adequite"), revs freely and with the right additions(decent cat-back and muffler) make all sorts of awesome noises(very ferrari-esque...no, Im not joking). The brakes are very good with a lot of feel. Scuttlebutt says that because of their size they tend to go away after a few hard laps, but I imagine that can be combatted with cooling, track specific pads and good fluid.
Aftermarket: Kinda light. In the mazda world of this era you didnt get noticed unless you had a miata or RX7. There are items available but youll have a choice of 1-3 items instead of 5-8 items.
LSD: quaife or OPM (The I4 or V6 uses the G-type transmission with the 10 bolt diff like the protege).
Struts: european application koni SA yellow inserts, tokicko blues and illuminas are available for the car. Further along there is AWR(both bilstien and koni versions), tein or K-sport. Even further than that there is an application(for the protege) from Moton and JRZ. The MX6 is basically the predecessor to the last gen Protege so many or the parts transfer wholesale without modification.
Headers: (V6)Hot shot, pacesetter or ebay hot shot clones. Unfortunately there hasnt been a modern header design for the car since hotshots in 1998. Its a good item with good midrange punch, but a 10yr old design now that hasnt been changed much at all. (I4), Im not too familliar with the headers for the I4, but its (again) the predecessor of the lump thats int he newest protege. A lot of parts apparently transfer and will fit.
Autocross..
GS: My personal opinion of the car is twofold. The car has won in the past and has shown its worth(the I4 more so than the V6). However I havent seen a modern GS build for the car. Times change and people learn things and have more resoruces available to them. For example many of the old winning cars ran stock rear struts and Illuminas on the front using stock or disconnected front swaybars. The tires at the time were the cambered BFG R1s and with an open diff this made sense in a way. Today this setup simply wouldnt work for a strut suspensed car at all. Will a more modern build(revalved konis/bilstiens, big front bar, H00$iers) close the gap to the current class leading Minis?... I dunno. For sure youd be a royal pain in the ass to them, but I think the car would still be 2nd tier at best.
STS: not even worth the time to think about this one. Car too heavy, tire restriction to much, "worse" suspension than class leaders. Um, no.
STX: more intriguing. 2 less wheels to drive makes the car a lot lighter than the class leaders. The chassis is very capable, and LSD would get things moving in the right direction, better wheel width for the car. Theres only one thing...nationals has moved to a much less grippy site. With the surface dictating a maximum of mechanical grip and tires that are limited on that, all wheel drive will be even more of an asset. Locally, sure bet, nationally? Dunno, have to see how things shake out.
DSP: Lots that can be done here. The wide/15" Hoosier can fit. Engine management and ITBS would release a lot of power in both engines in my opinion. You would be up against the class leading BMWs and non-turbo subarus. On a slick surface the drivetrain advantage would be 4WD>FWD>RWD (driver excluded). Still second tier stuff anyway you want to cut it. Built and driven well I dont see why the car wouldnt be able to trophy (except the well driven BMWs, 4WD subies....and the 1st gen MX6 with unlima-boost and more update/ backdate to take advantage of

).
Just some random thoughts and idle ramblings.
Contact user "Inspector 24" as he has a well put together 2nd gen that he is campaigning in STX. He also takes it for spins at Miller Motorsports Park and can add(or subtract) from the things I have mentioned.
Finally....get a Miata and a rollbar and forget everything I said above. I like the MX6 a lot as it has a lot to give, but really... In terms of accessability, servicability, expense, aftermarket, resale(this one is big..theres markets for stock AND modified miatas, not so with modded MX6s), available paths of competition/competativeness..this is a no-brainer. Oh yeah, RWD miata crunchy goodness and a suspension that works? Wait...I forgot what the question was.
Gavin