Many brakes have holes that are drilled at an angle such that escaping gas is directed backwards. The effect is like a jet engine. High pressure gas exiting in a rearward direction provides a corresponding thrust forward which alleviates some of the recoil forces directed back towards the shooters' shoulder. Due to the speed of the exhausting gases, I believe only a limited amount is able to enter the holes, if you add more holes, eventually you will come up with diminishing returns and there will be no more reduction in recoil. If you had more holes than metal, I believe the recoil reduction would be the same as a smaller number. If you cut the entire section off, you revert back to unported recoil because now there are no more gases pushing the gun forward. A shorter barrel will recoil more due to the loss of weight but unless your are talking very extreme changes in weight like a 28" cut back to 20" I doubt you'd be able to feel the difference. These are all just my opinions based on my experiences and I'll not take offence if someone else knows more or finds error with what I've written.
NoCAL
