If you didn't overheat anything then I'm guessing it was doing what it should. The factory builds in a lot of "safeguard" into the trucks so they can pull like that. I'm betting those were some fairly steep inclines with little run up to them??
A performance exhaust would of helped somewhat by keeping the EGT's down and flowing a little better, but I wouldn't put a "hot" tuner to it with that kind of weight.