The widest you should be able to run comfortably is 265 front and 285 rear.
Any larger may be too big and decrease performance.
Track / Autocross setup: Forgestar F14 (21.8lbs & 23lbs)
Front: 275/35/18 Continental slick on 18x10, Rear: 305/35/18 RA-1 on 18x11
Drift Setup: Front: 275 Continental slicks, Rear: 255 Michelin PS2 street tire
Laguna Seca previous record - 1:49:498