The turning radius limiter is only included with the Handling package. If you do a straight across swap then you won't have the reduced turning. I believe that PowerSurge is the member that put the 18's on his non-handling package car and could not create any problems of rubbing.
As for road noise, it will definitely be worse than the 16's, imho. I had a SEL with 17's before my Ti, and I was really surprised at how much louder the 18's with PS3's were. The tire that you choose will be your main factor in it, but no matter what the reduced sidewall is going to cause noise to be more easily detected.
I am partial to the 18's, fwiw