Payload was really the wrong choice of word here as that would generally be taken to mean only the shot charge. Total ejecta would be more appropriate as this includes shot, wadding & powder gases, all of which are ejected from the muzzle. Increasing this weight or increasing the velocity at which they are ejected increases the recoil. Ejecta weight can be increased without increasing recoil "IF" the velocity is dropped by a balancing amount.
Personally I don't think a really scientific study has ever been performed to establish just what increment of increse or decrease is required for most shooters to actually be able to feel the difference.
MANY GIMMICKS are pushed on this fact, they can not really be proven wrong, But by the same token they can't prove they are right.
Keep in mind though that if anyone claims they can do something to your gun which will both give an identical load an increase in velocity while simultaneusly reducing recoil they are either totally ignorant or a flat out liar.
