First - to reiterate, yes, we are in DSP.
Second - Poly mounts are legal in STX, and so is HP Tuners. You can't change the injectors in ST, but you COULD get the stage 1 flash to get the higher redline and retune w/ HPT to keep your 42# injectors. As long as you run premium fuel only, you should be fine. Regardless - you don't have to run in SP if you don't want to just becuase of poly mounts.
Third - I think you'll find that tire idea not as swell as you think. Try it - you may be right and I wrong - but that's a LOT of tire under a car and almost all of the extra width will be outside. That's good - except it changes your scrub radius DRAMATICALLY, which will make the cars turn-in awkward. Also - hard to tell how the EPS will handle it. I really think that lowering the cars CG and tuning the suspension properly in conjunction with 245/45/17 rubber would out-handle a stock ride height on really wide tires. Maybe not out-grip, but certainly out handle. Autocross is a grip race - but it's also a handling race. If it was an oval with constant radius turns, sure - but it's not. I think you'll learn to hate slaloms with those shoes on. I could be wrong though... Oh - and your math is WAAAAAY off. a one-inch increase in radius - almost 2" overall diameter - is a LOT more than 1%, lol. In fact, it's almost 10%. I'm all for gearing it up a little for 1st, but teh VAST majority of an autocross course is spent in the middle of 2nd. I wouldn't want to lose that much gearing in the sweet spot to gain a tad more throttle at exit, dude. Modulate the throttle better, lol. A rear sway and stiffer springs - a good bit stiffer in back - will increase the effectiveness of the stock LSD many times over, so put some trust in it. Or try a very light left foot brake action with the hammer down leaving a corner - works wonders for getting teh LSD involved if you're getting too much spin. Oh - and more negative camber, too. I've got -2.5/-2.7 on stock suspension on 245/45/17 Hoosiers, and it's not enough according to the pyrometer. If you're not taking tire temps, you don't know if your camber is right.
