'Just guessing.... I'd expect that the VG's change the lift characteristics of the wing (usually their purpose), perhaps spanwise, as well as in total. The VG's may also change the aileron effectiveness for low speed control. So the wing bending loads could be changed, allowing more of the load to in the fuselage than originally intended by Piper, as the wing bending loads have been reduced a bit by the VG installation. Thus the VG manufacturer can claim greater payload. But, it's my guess, I don't know this VG kit.