I'm running 275 on 9.5" square and so is M3adjuster. The square setup reduces the understeer that's associated with the stock tire setup.
Is your suspension stock? If yes, square is the way to go because the front camber adjustment range on the stock suspension isn't sufficient and you'll be eating up the outer edge of your front tires.
2011 E90 M3 ZCP/AW/Ohlins R&T