The bigger brakes really don't matter for autocross. In fact, a lot of the CSP guys "backdate" to the smaller 1.6 brakes to save weight. In fact, they do EVERYTHING they legally can to save weight, often including the removal of factory chassis braces in addition to the usual A/C removal and such as that.

So, that leaves you with the torque... and how you can apply it. But, there may be other things to consider.

There was a short thread on Miata.Net earlier this year raising the same question. And another thread about "bench building" the perfect STS2 Miata.

STS2 allows you to change wheels. Personally, I'd go for the lightest possible 14" wheels and 205/60 Azenis at this time. OR... for better tire selection, maybe some ultra-light 15's with up to 225's. But weight matters, always keep that in mind.

STS2 only allows an OE VSLD, and there are those who will say that "any LSD is better than no LSD". I can't say, myself as my experience is mostly in my own car that has a Torsen. (and I sometimes run STS2 if I know my competition doesn't mind... it's that or get eaten by CSP)

The 1.6 cars are a little over 100 pounds lighter than the later cars. That can make a difference, too.

And the 1.6 cars are geared for better acceleration, which helps balance out their having less power. This is why a lot of people feel that the 1.6 cars feel "peppier" than the 1.8's.

Lots of stuff to consider, for sure. If I were starting from scratch with the intent of building a car for this class, I'd probably seek out a nice 1.6. Should be able to pick a really nice one up for under $3k... or a ratty one up for well under $2k... or a fixable wreck or terminal mechanical failure for well under $1000. Put a good suspension under it, good wheels and tires, fresh and functional VLSD, and go have some fun!