265/75 deviates about 7% from stock size, 285/75 about 11%. Acceleration and braking will be affected the same amount.

I was very happy with the 265/75s on my old 99 Trooper, never felt tempted to go larger, not wanting to lose more power. I went wheeling at Moab several times with those tires and the OME lift. You don't have to have the lift to fit those, but I think it is an easy and worthwhile mod. A pair of OME coils for about $170, adjust the front ride height to match, and you will be an inch taller, AND have better load handling, more controlled ride, and even a little less body roll. Very easy install, too.

IMHO if you are doing any serious off-roading, then an MT is your best bet, and the MT/Rs are a good choice. But for light or infrequent off-roading an aggressive AT is going to be fine and will last longer, and be quieter, and usually better in the snow, too.

If I were you I would look at BFG ATs or Nitto Terra Grapplers...as ATs go they are pretty aggressive, relatively long lasting, and good on road. I have owned both and would not hesitate to buy either again. I think the Nittos are a little cheaper generally.

Here is a pic of my old 99 Trooper in Moab, sporting 265/75 Pirelli Scorpion ATs and a 1" OME lift.