You have to take into account what the TW will be after you load 2000lbs of motorcycles in the rear. The 980lbs is dry weigh. I was thinking he would be too light. With a Max loaded trailer weigh of 8800lbs 900 to 1200lbs TW would be the ideal recommended 10 to 15% TW. I would probably add a couple of extra batteries for additional TW.
I read for the 2015 sierra has a 2250lb payload...
"2015 Chevrolet Silverado/GMC Sierra, which keeps its 12,000-lb max tow rating for 6.2-liter V-8 models with the Max Trailering Package. Max payload for the Silverado and Sierra tops out as 2270 lb, for Double Cab models with four-wheel drive"
