wow, 8 year old thread brought back from the dead.
Yes you can run 18's all the way, I do it for the street and racing.

Hope you're still up on this site. I have the opposite question for you. I'd like to run the back set of tires and rims on the front, all 4 being the same size as the factory tires and rims. Is this possible without any damage, modifications or serious of handling?