I'm with 500gr. on this one. As far as a percentage of recoil, I can't give one, however they do reduce the sharpness of some gun's recoil. Taking the bite, is as good an analogy as any. It definitely is superior to a muzzle break due to not increasing the noise. Shooting without any recoil reducer for me, is better than a muzzle break.
: The various heavy 'special' pads also are good reducers of felt recoil. In conjunction with a murcury reducer, perhaps better than a standard rifle/pad with a break.
: My experience with them is minimal, but made my .458 almost pleasant to shoot. With it's heavy barrel, the reducer balanced it out nicely. This was back in the 80's.