Voodoo, I do have my 870 and it hardly moves up at all.
Just because you have to pump the gun does not mean you will lose your sight picture, but if muzzle is up in air high you definitly will. My gun stays basically level even at high [for me] speed with my ported 870. Have had a witness say barrel didnt even move upwards noticably. Works for handguns and the muzzle flash from shotgun looked a lot worse than anything comming out of the ports.
If I ever dust off my nikons I will shoot a couple at night with long exposure of ported and unported and try to post the results here.
Posters here should tell all the rifle guys how important the flash hider is on the rifle forum, as they seem to be just fine without them, and not willing to pay the pre-ban price. BTW they shoot trap at night out here and dont seem too bothered by the porting, couldnt see bird after first shot if it was that bad.
IMO I couldnt tell differece, saw tartget as I always do.