I picked up my 2500hdAT4 gas on August 15th, so far very pleased with all aspects of the truck. Took about a week to get used to the size and driving characteristics from my 2014 Sierra 1500, but nothing major, parking and narrow roads are fun.
I took the truck on a long trip Monday 8/31 about 600 miles total to the top of Mt. Washington Auto road and back. It got a respectable 15.5mpg all around. I thought that was fair given going up an 8 mile auto road was steep and coming down I was in low gear. The truck rides good and is very comfortable.
I just got back from towing my travel trailer for the first time, it is light for this truck at 7700lbs fully loaded, so i would say arounf 55-6000 with all of stuff in it. Truck was confident had plenty of power and had awesome grade breaking compared to the 1500. I got around 9.4mpg for the entire trip, which is ok, i saved 10k from getting a diesel so i knew fuel cost were going to be more.
Overall after 1300 miles in about a month I love this truck. Not a lot on the road so it feels unique, gets a bunch of looks and it will be pulling a fifth wheel in the next 18-24 months hopefully. I feel full confident in it's ability, the extra weight and added payload and power are awesome. Like you guys said a mild tune and it will be unbeatable.
Mike