I autocross, and I can tell you that a wider rear tire on a mustang is a mistake. these cars are proan to understeer as is and a wider rear tire makes this worse. If its any indication to you I'm running 17X10's this year and I may still go with a 275 series tire. I ran 275's on a 17X9 last year. You want a wider wheel than an 8" unless you are only running a 245/255 up front. Also, if you plan to autocross/open track you will want the same size all around so you can rotate tires to get as much life out of the tires as possible. Staggered setups are not for autocross/open track in a mustang.