Here is my take on the whole overload deal. On my truck the overload leaf springs rested with an empty truck(my truck "empty" weighs 8200lbs) about 3/4" below the overload stop. When driving down the street, the suspension would only have to travel 3/4", before the overloads kicked in. What you have is a serious change in spring rate, The main spring pack has one rate, and the overload pack has another. So when you encounter a bump, the main springs start to deflect, and the suspension travels, now after it has traveled 3/4", the overload springs Are at there stops. If the suspension continues to travel, the spring rate now has to be, the main spring + the overload springs. The overload springs have a much stiffer rate then the main, this rapid change in spring rate results in the severe rough ride. This usually translates to a very sever kick, or slapping feel in the rear, which is anything but pleasant.
When I removed the overload spring pack, I allowed the main springs to travel without the hindrance of the overloads. Now when I go over that same bump, MY main springs deflect and the suspension is allowed to travel, without interference. This freedom of movement, and linear deflection rate result in a smoother ride. The air bags when the truck is empty, don't carry any real load, but then help absorb vibrations, and shock. This also contributes to the smoother ride.