We're out there a ways from the original question but I'll add another couple cents since the discussion went this direction.
I have the exact Brian Samson (sp?) butt pad/hook that was asked about by the OP. Actually I have a couple of them. One on a springer and one on a Veteran. After shooting both guns with OEM pads and then adding the Samson contraption, there is a DEFINITE advantage to using that curved buttpad, especially from shooting sticks where the gun is trying to slide down your shoulder without it. And there's the rub! These hooked butt pads ARE helpful or we wouldn't be having this discussion, whether for AAFTA Hunter class or for Extreme FT.
As for a true butthook like Mike N is referring to (one that goes under/over/around the armpit and/or shoulder, I personally see a benefit only when shooting from forced offhand shots. I only add it to the gun when shooting offhand (to my understanding, this is allowed in AAFTA Open class). Mine is a simple affair, just a straight bolt that threads into the bottom of the aluminum buttpad, near the bottom, and sticks straight back, under the armpit. For me it creates a sort of muscle-free anchor point so that the weight of the gun is leveraged between the non trigger hand, and literally "hooked" under the armpit. Less muscles required to hold up the gun, again, in my case.