The widest I went was 295 mounted on some 18x10's. I have heard of people having rubbing issues with 315's on lowered cars. Some people suggest getting the fenders rolled, some people say their's rides fine.
^ Clearance for such a spacer depends on the off-set of wheel :yes
Just installing a spacer without knowing if one has the clearance to run one is a mistake. Running a 315 requires a 10.5" wheel and rubbing is hit and miss. As mentioned, some rub and some do not, so be prepared to address some possible rubbing.