Lengthen forcing cones - does have some effect. Could detect the difference 94% of the time, BUT could not quantitatively evaluate it. Didn't feel softer so much as different.
back Bored barrels. - no effect on felt recoil that we could tell. Contrary to popular belief, with some loads, did get a slight velocity increase.
Porting - ported chokes - zero effect on recoil in a shotgun. Some people do believe they can get less muzzle rise with the right porting.
Weights - fore & aft (types of weights) - more weight will definitely directly affect felt recoil. Newton was right.
Various types of piston/spring recoil reducers - Every one I have yet tried did seem to reduce recoil, but almost every one of them add weight too, so small surprise. Benelli's two piece stocks work with no added weight, but I hate for a gun to not behave like a one piece entity.