I am assuming that the wheels Troy is considering are 911 offsets ie; ~55mm front and rear. That is what limits the front tire size. It pushes the tire too far out of the wheel well.

Even if you go with cast centers, as I believe mine are, getting the correct "928" front offset is sure nice. Once you are spending $2000 on wheels, is another 3-400 too much to get the right offsets?

My 928 with the old 18" wheels in 911 offsets did drive just fine. I didn't have any tracking problems or any other ill effects. They do work.