I have a Smith & Wesson Performance Center 629 (the one with the 7 7/8" bbl and integral Weaver rail machined on the top of it) and it is factory Magna-Ported.  The bell of the objective lens is right over the front sight and ports, but it doesn't even get soot on it.  I expected a nightmare trying to keep it clean and functioning, but there is absolutely no effect whatsoever.  Obviously the S&W "Power Port" would be another story.  With handloads it shoots into about 2" at 100 yards.  Nice deer slayer, but I haven't killed one with it yet.