I've got a 29-2 frankengun with a 6" classic full underlug barrel that's mag na ported and also a 66-3 4" mag na ported gun. Both shoot very well and I can definitely see the venting of gasses when conditions are right.
I'd say that the porting does work but it doesn't turn a magnum into a .22.

It is a little more to clean around the ports but not a biggie.

I wouldn't pay a premium for the porting but I would definitely buy a gun with mag na porting if the price is right.