It depends on if you want to lower the car, and if you can keep your wheel offset close to stock. A 235 should fit without any modifications, that's what I have on an 8" +48mm wheel. Rolling the fenders is an option for added safety factor.
A 245 has been done pretty commonly, so that would work - I would definitely recommend rolling the fenders for that tire.
Apparently Cobb is currently running a 255 on their car, with their suspension. Personally I'm not sure how they accomplished this, as I've heard that even 235 tires rub in the back with their drop. Using a high offset wheel to keep the tire away from the fender isn't possible with a 255 cause you'd probably run into the dampers.
IMO, a 235/40 is the best tire, a 245 should be acceptable, and a 255 is pushing the envelope.
