I've got a '15 GMC dually, and put in the lower Stableloads as well. I can only get 2 wedges in with the truck empty and sitting; I also have about 1/2" of clearance between the spring pack and the overload to work with.
With my AF992, the truck sags 3" with 2 wedges in, 3" without. They may make a bit of difference in handling, but unlike my '11 GMC dually with upper stableloads, the sag is too much. Actually, the '11 didn't sag more than a couple inches with the same camper.
I'm going to put air bags on as well, so that I can easily raise the truck, use all 3 wedges, and adjust ride height. With a 5000 lb+ camper and a 5200 lb boat/trailer on a 36" Supertruss extension (no option for a weight-distributing hitch on the trailer), it's just too much sag and really affects ground clearance on the extension.
- Clay