I have 35"s with just a front 2.5" level. Looks good, drives good, and power loss is really a non-issue. I also have 3.21 gears and was worried about going too big/heavy with tires, but was pretty surprised just how little it affected things. I actually have 295/65r20 on stock night edition wheels. Comes out to 34.75x11.8 I think. Yokohama Geolandar MT's.
I have a 21 Lonestar edition. Which is the Bighorn with night package and 12" screen, LED lights, and a few other higher trim features. My truck is light compared to higher trims, with 1660lb payload or something. My truck was surprisingly quick stock, and is still pretty quick now considering I'm level with bigger tires. I suspect your truck is equally light too.
0-60 is slightly slower but 40-70 feels just as quick as before. Just this weekend a lifted Chevy RST 6.2 tried to "race" me on the interstate, but I just slowly drove around him at like 90mph. Power to weight, etc.
My mileage isn't terrible either. Average 17.5 now. Calm highway driving I'm seeing 18.5 to 19 at 70mph in 8th gear.