We added them not for the weight but to get us away from the overload spring. When loaded we were 1/4" off the overloads so when we hit a bump a got a double bounce as the suspension bumped off the overload springs. The air bags leveled us a little and got ride of the second bump. We run about 5-8# empty and the ride is a little worse but made a difference while loaded. I set the bags about 15# then set the trailer on it. Have not measured it with the trailer attached. Some people air the bags up separate to allow them to level side to side. I just "plumbed" them together so they equalize like on my big truck.
Would love to just swap out the suspension for an air ride suspension but not in the budget.