When I bought my GMT800 new 10 years ago, GM offered two performance cat back exhaust systems through the GM Accessories catalog. Both were made by Corsa.
I still run the factory exhaust, since I want every dime of use out of it that I already paid for with the price of the truck, but one thought stuck with me...
If Corsa jumped through all the quality hoops and passed all the standards tests that GM typically subjects and holds its suppliers to, then I should keep those two particualer Corsa products in mind should I ever need to replace the exhaust.
As it is, I don't think GM left much on the table in terms of any performance gains with their factory exhaust. One can get something else that will sound louder, but in terms of a real world balance of torque/power and quiet/sound and longevity/quality of materials, the stock exhaust is hard to beat.
It only loses significant ground to the aftermarket in the categories of looks and bling.