Forgive me, but I see nothing in Torklifts description of stableloads to indicate that they are marketing them to be a "replacement" for airbags or additional overload springs, but an enhancement to stock suspensions in the area of stability.
QUOTE: Source, Torklift web site.
Your vehicle factory suspension was designed by the vehicle engineers to carry the payload on the leaf spring suspension.
Overloads such as airbags can help a vehicle that is towing or carrying a heavy payload by keeping the vehicle level.
Leveling improves braking by shifting more weight onto the front wheels due to a forward shift in the center of gravity.
Airbag overloads can also help by keeping the vehicle front alignment closer to specification as again the vehicle is closer to a stock level stance.
As stated above, the StableLoad dramatically improves vehicle handling characteristics by "reactivating" the stabilizing affect designed into the rear springs by keeping them actively under load the way your vehicle factory engineers designed them to operate. The StableLoad accomplishes this and functions equally as well when used with or without airbag overloads. For vehicles that do not yet have airbag overloads we recommend starting with the StableLoad prior to investing in airbags.