Aside from FS, STX is definitely the cheapest way to go, but I do
think you'll have a tough time being competitive with that setup in our
region - there's just too much weight and too little tire. With 10-11"
wheels and 275mm tires in STU at least you're giving yourself a better
chance, and you could always run ESP in street tire. Whatever you do,
don't skimp on wheels and tires.
Also, typical lowering springs
are going to be way too soft for autocross. Stiff springs and shocks
that can control them are important. This is an area where a lot of
people half-ass car setup. Another is with alignment. I'm running
almost 4 degrees of negative camber up front, and I personally align my
car to some pretty exact specs.
My advice to make your car more fun and much more
competitive: Stay in FS for now, but prep for that class. If you
decide to move up you won't have lost much, if anything. Get a big
front sway bar, a cat-back exhaust, and most importantly, good tires.
I'd go 245/45 since your wheel is only 8" wide. Find out what good
stock-class alignment numbers are and get it aligned to them. Then show
up and win because you've got a car that's competitive in its class and
is well-prepped.
Mississippi Region #83 SS / #97 STU