I would personally not want a 255 in the rear... but that's just me. And considering your mods, I would not even consider a tire as "skinny" as a 255 for the rears. Besides, having seen (and planning on running in spring) a 275 on a 9" wide wheel (with the correct offset) looks really nice and meaty. If you had to choose, DEFINITELY go staggered. 275 in front = poo. 255 in rear = poo. 255 in front and 275 in back = hot.