On the subject of Owner's Manuals --
The online
2012 GMC Sierra Owner Manual states the maximum tongue weight, when using a weight carrying hitch, is 600#.
Assuming a TW% of 13%, the loaded trailer weight would be limited to 4615# if not using a WDH.
The 2012 manual also specifies:
When using a weight-distributing hitch, the spring bars should be adjusted so the distance {ground to wheel well edge} is the same after coupling the trailer to the tow vehicle and adjusting the hitch.
IOW, the 2012 Owner's Manual implies the WDH should be adjusted so the front axle load restoration is 100%.
Ron