Well, you won't be able to run any less backspacing than you have now if that's the case. A 1/2" to the quarter is pretty much the minimum clearance you can have without rubbing. Unfortunately, with a 15x8 and 5.5" of backspacing you should be out of room to the inner tub too. I certainly can't see you running more backspace, and with only a 1/2" to the quarter that's exactly what you'd have to do in order to clear 275's. It sounds like you're already running the widest tire you're going to be able to run without a mini-tub or cutting/rolling the quarter lip.