I don't see anything (yet) on SuperSprings' website that says their product "results in additional load carrying capacity."
Here's a description of the SuperSpring, as quoted from their website:
SuperSprings are easily and quickly installed directly above the vehicle’s rear leaf springs. They work in parallel with the vehicle’s existing suspension system and “kick-in” on demand. The increased spring-rate produced by installing SuperSprings helps eliminate vehicle rear-end sag and improves towing capabilities. SuperSprings unique anti-sway properties reduce body roll providing enhanced driver safety and control. These features are most noticeable when cornering, encountering strong cross winds or dealing with wind buffeting from other traffic.
Sopunds pretty similar to Torklift Stableloads, in that they (SuperSPrings)
"...helps eliminate vehicle rear-end sag and improves towing capabilities."