Freon, what's your alignment, particular camber currently?
I've been running 255/35-18 square for a while with a 3mm spacer in the front. That seems to work very well.
I'm highly considering running 275's in the rear next, but I am concerned about the fit. My rear fenders are rolled though so maybe a 3-5mm spacer in the rear would make this work well?
Let us know how the next autocross goes