Boost, offset is the most critical factor when lowering. A difference of 3mm offset may make the difference between rubbing and not. I am running 18x8 +35 with 235/40/18 and 18x9 + 35 with 265/35/18. The rears are very close to the outer guard (nothing rolling won't fix), but due to stock ride height, they don't rub. The fronts are absolutely fine, could be dropped a fair amount. Be aware though, that a 18x9 +35 might sit differently to a 17x9 +35. Not sure if that is really helping, but my guess would be if you are running 255s on a 17 with the correct offset it should be OK.