I hate to say it, but these posts about what kind of mpg drop will I see are getting real old when talking about putting bigger tires and lifting a truck. I mean you bought a $55,000 truck with the largest engine possible and you're worried about the .2 mpg drop you're going to see between a 275 and 295? Pretty much tells me you can't afford the Denali if you're already worried about what it will cost at the pump.
/soapbox
But to answer your question which is confusing because you talked about narrowing it down to Toyo AT2s but then said it was a different charge from the Nitto Terra Grapplers (???). Then you talk about the difference between the LT and XT, which in Toyo AT2s, is the same tire. There is no difference. Where the difference is is from the regular Toyo AT2 and the AT2 XT. The AT2 is a 8 ply, passenger rated tire, whereas the AT2 XT is a 10 ply, LT rated tire. The XT is heavier and stronger built than the regular AT2. The AT2 XT also has deeper tread and larger sidewall lugs than the regular AT2. The XT will hurt your gas mileage since it is heavier, but like I said before probably only by .2 mpgs. If that's a concern for you, then by all means get the AT2 regular instead because we'd hate for you to have to spend an extra $5 at the pump to fill up your full size, 4 wheel drive, large displacement truck.