I run a 295-45-18 in back. There are some who have gone 305 and beyond.
It's a combination of weight and the live rear axle.Porsche and vettes stagger their wheels, right? They handle awesome in the corners. How come a mustang can't? I would think it has more to do with the suspension than staggered tires.
Then again, I've never driven a stang with a staggered setup, do I dunno.