The new GM truck can fit larger tires than prevous without rubbing. On my 2012 I am running 305/55/20 (33.5") on factory 20s with no rubbing.
I hated the look of the 3" of rake from the factory so I put 6-turns on the T-bars which raised it 1.25-1.5". It still has some rake so when I tow I am perfectly level. There was no affect on ride quality but if you go higher or start using aftermarket keys ride will suffer. If fully leveling you really need to go with longer shocks and the Cognito UCA's to save your ball joints
I wouldn't go full level unless I had airbags. Towing with the Cali lean may look cool to some but its a not the way to go IMO...