Like some of the guys said, get the correct offset wheel. If they are sticking out too far for whatever reason, they will run a higher risk of rubbing the outer rim of the fender. Also make sure not to drop the car redically. Not that this really needs to be said, but dont heat or cut the springs to drop it, use some good springs or coil overs to get the effect. If you do this you should have no worries of any rubbing from 18 in wheels.
Drive a TT and you'll realize that turbo should be a standard option on all cars!!!