There are some people that run outrageous tire sizes, ive seen a 315 on an 8.5" wheel. Not saying that's safe, but Ive seen it, and it looks terrible. It really all depends on your preference. Most guys like a flush look, some people like a little tire bulge. Back when I had a set of 8.5 rear wheels, I put 275's on them, which bulged, but not to any degree that was unsafe by any matter.
9.5 is a rather odd wheel width when it comes to mustangs, but that's not to say you will have any problem with mounting or finding tires for them. Most guys run a 9" wheel, or a 10/10.5 in the rear. On a 9" wheel, a 255 tire will give you a flush fit without stretching the tire. Anything above a 275 on a 9" will start to bulge a little bit, so I would guess a 285 or 295 would be pushing the limit on a 9.5 if you don't want an extreme ballooning look.
