Remember... build tolerances on Mustangs can get kinda sketchy. Also factor in to the equation what brand 315 you will be running. Some run wider some narrower. I am Running Sumitomo 315's with a 1/4" spacer and H&R SS springs and all i had to do was flip the quad shock. No rolling of outer fender, no pounding of inner fender. No rubbing what so ever, no matter how hard i corner. However i have taken everything out of the trunk and run a rear seat delete so the rear kinda looks like its at stock ride height...lol.
As for traction... its more about compound than anything else. Rather than try and stuff a 335 under there... save up for some KILLER compound 315's. In my case, As soon as the Sumi's run down its time for some BF Goodrich KD's.
For example: A super nice compound 275 will kick the crap outa some cheap 315's. However, 315's look killer