Well, if I had my 870 I'd be able to tell you! Unfortunately, it's at Vang Comp getting "ported". Hans Vang estimates that his porting system reduces felt recoil by about 15%, increasing accuracy and reducing recovery time. I supposedly also reduces muzzle flash (though I don't see how).
Several of my LE friends swear by his work. Who knows?!?!?
He has lots of info on his site - check it out: www.vangcomp.com