265/40 M/t DR is bulged out pretty good on the stock 8"
255/45/18 would be the max IMO, same as the GT500 fronts, they look too small on the GT500 9.5" wheels and would probably look OK on the 8s. You can get F1 goodyears for cheap but it will still probbaly look short compared to the 235/50 fronts.
I am probably going to give up on the 10" rears and pick up a set of the 18" 2010 GT500 wheels (9.5 wide) with the F1 225/45 front and 285/40 rear. I had them on my 2010 GT500 vert and I like the wheels.