Please don't call it the "protouring look." Protouring is about function, so we put on larger rims, but only to clear larger brakes (and I don't know any runnimg 15" brakes). So most run 17s or 18s.

That large a rim requires a very short sidewall making it hard to "feel" the limits, and forget traction in a straight line, or powering out of a corner (hence the drifting crowd runs ultra short sidewalls). It's more like "proshow" or if you raise the car a few inches, it's a donk.

A 275/30R20 should fit on the rear with a 9.5" rim and 5.5" backspacing, but it will be tight.

A 2" stagger is too much stagger and look even more goofy. I'd suggest, if you really want to go that big, do a 18/19 combo. Max on the front would be a 245/40R18, with 275/35R19 on the rear. My real preference would be to do 18s all around though.