HD-J78,
I ran my stock tires untim bald. Then put on 316's, (Nitto Dune Grapplers) and again ran them until needed to be changed. I now have 35" Dick Cepek Fun Country 11's and love them. Noisier than stock/Dune Grapplers, but not too loud.
With both the 315's and the FC 11"s there did seem to be a slight decrease in power when starting out, but not noticeable otherwise. No real differences in mileage. . but I do watch my inflation pressures pretty closely.
You will need to lift the front end if the 35's you choose have any significant width... they will rub. I went with KORE 2" inch levelling kit, (with heavier snow plow rated springs)/extended sway bar links and longer Bilstein shocks and love the ride.
Eddie