I went from 31" AT's to 32" MTR's and saw mileage drop a full 2mpg. They also wear considerably faster....In Arizona, the rocks kill tires before the tread is gone, so I can't attest to how many hwy miles you'll get out of them, but I wouldn't expect anything beyond 35-40k, uless you run tires down to bald.

How much trimming you do depends on the rim you run. If you stay with stock rims, you'll probably get by without much. If you go with any more offset, plan on doing significant trimming to the front fender and pinch seam. Mine also rubbed at the top of the rear fenders as well and had to be trimmed there too. This is with a suspension lift and 15x8 rims with 4.5" backspacing.