I had a .375 H&H rifle Mag-Na-Ported, and it made quite a bit of difference - I'd estimate a 20% reduction in actual recoil and 50% reduction in muzzle jump.
As for shotguns . . . I've fired Pro-Ported shotguns and I can't tell the difference.
Think about it - the powder charge of a shotgun relative to the weight of the shot column is tiny compared to the powder charge of a high power rifle and a bullet, so the powder contributes little to shotgun recoil. Add in the pressure factor - CHAMBER pressure on a shotgun is comparable to MUZZLE pressure on a rifle - and by the time the shot charge passes the ports, there aren't enough powder gasses at a high enough pressure to make much difference by porting them upward.
Oh, I suppose theoretically there might be a little reduction in muzzle jump . . . but not enough for me to notice.
