how much sag is too much

I have similar TV and notice a slight amount of sag as you have experienced, but out west where the wind can blow pretty hard plus trying to maintain speed on these 75mph interstates, I wouldn't even consider leaving home without my WD/SC hitch engaged. Love the solid feel of going down the highway and experiencing virtually no sway even when being passed by semi's, have traveled a short distance without WD one time and towing comfort level really wasn't where I wanted it to be. Enjoy your Silverado, ours pulls great.