Well, in the three years I've owned this Sierra (Red/Silver), the three heaviest things I've towed were a 1995 Chevrolet Caprice (~55 miles), 1998 Honda Odyssey (~8 miles) and a 1993 Chevy Astro (~10 miles). Did it all with no problems.
As far as actually hauling....I'd say the heaviest thing I ever hauled in it was a ~500 pound solid walnut executive desk. Did it all in stride, didn't even know I had it in there.
My old GMC (solid red)....God knows how much heavy stuff dad hauled in it.
The heaviest thing it ever towed (keep in mind, 14 bolt rear, 4.3L V6, and the NV3500...) was a Case IH 7250 tractor on a 30' flatbed trailer. The tractor weighs 18,500 pounds, and the trailer (I would think) would weigh at least 4 tons.
So we're talking about a C2500 with a 4.3L and a NV3500 towing a load weighing 13-14 tons. It was about a 100 mile trip, but it had to be done. By the time the truck was done, dad decided to get a new 2000 Chevy K2500 with the 5.7L.