It all depends on the offset of the wheel and the actual running width of the tire. Plenty of people have fit 275/35/18 Michelin Pilot SS on the rear with a 18x9.5 ET58/62 from Apex in the EC-7 and ARC-8 wheel models. When you get into VMR fitments (18x9.5 ET45/50) it gets significantly harder to fit that wide of a wheel in the fenders without a serious roll/pull. I have been told that, if you have the right offset wheel, you could even go 18x10 and get a 285 in there, but then wheel diameter differences start to get a little screwy.
Fronts max out at about 235/40 with an 18x8.5 ET45 both inboard and outboard AFAIK without serious camber adjustments.