Well, I have worked everything I own to the max...
03 4.8 tahoe, pulled 7k lbs 100 miles on and off, did it like a champ, took forever to get up to speed though, lock it in 3rd gear and let it eat at 60 to 65 mph...
2012 suburban 3.08 gear, loaded to 7k lbs and 800 lbs inside, was a rough pull, 6 speed helped but hills sucked unless you had it about 3k rpm then it would motor through.
2011 Silverado 6.2 did everything well pulling 6 to 8k... did many 1000 mile trips towing in that truck...
2011 yukon xl denali, so far towing with it, 5k roughly and 6.2 performs almost as good as the silverado, I do have max trailering etc on this, 3.42 gears so far so good.
Few things to take away from my choices. If I was pushing 8k tow regularly I would go to 2500... Just for the beef on the rear suspension and bigger brakes.
Secondly I think the 6.0 is more of a powerhouse grunt than a 6.2 but that's my take. LASTLY... TRAILER BRAKES. Tandem axle trailer with that weight should have 4 wheel brakes... NO EXECPTIONS>>>>> this saves the brakes on the tow vehicle and make damn sure you don't have a el cheapo supremo brake controller.
Properly balanced you should be able to tow heavy without issues as long as your not beating the hell out of things.
I have days where I want a 2500 just for the extra beef.