You don't port a shotgun barrel to reduce recoil. It isn't going to happen. Barrel wall is too thin to offer enough surface area for the gases to push against and the pressures are too low to offer a significant decrease in recoil anyway (see below).

You port a shotgun barrel to reduce muzzle rise, allowing the shooter to acquire the second or third target quicker. The gases vent through the top of the barrel and push the muzzle down.

Although the noise levels are higher in a ported shotgun barrel it isn't like the sharp ear splitting crack of a braked rifle barrel. A shotgun is only operating at 10,000 - 11,000 psi where as a high power rifle is operating at 55,000 - 65,000 psi so the expelling gases are moving at a much lower velocity inside a shotgun barrel. Also by the time the gases reach the muzzle the pressure levels inside the barrel have dropped to just a couple thousand psi.