I just finished my second autocross event today and I have a Roush suspension with 18x10 wheels with 275/40/18 Nitto Invos. I would not recommend going wider in the rear than what you have in front. My car handles a lot better than stock but still understeers quite a bit, so going with a smaller wheel in front would make it worse. It is nice to be able to balance the car with the throttle, but more front end grip would help a lot. I am switching to Nitto NT05 in the spring, hoping for a little stiffer sidewall.