I believe there is a big difference between Magnaporting and a muzzle break. Has any one bothered to read the claims on the Magnaport site? They talk about felt or perceived recoil. http://www.magnaport.com/sgun.html

I have a Marlin 1895 45-70 with 26" octagon barrel ported by Magnaport. I was shooting hot lever action reloads. I could not tell the difference after the port job. My final solution was to drop the load down a few grains.

I have a custom Mauser 375 H&H Ackley Improved with a 26" heavy barrel. I had it built with a removable break. I can certainly tell the difference with the break on and off.

Given the porting I have seen on shotguns, I would have to say that I doubt it would make much of a difference. I think stock fit makes more of a difference.

http://www.damascus-barrels.com/Movie.html