I've just ordered a prefit Limbsaver for an 835. The slip over model seemed to goofy looking to me, and I am sure I could loose it off the butt w/o to much trouble given my luck and ability to mess up.
What little online checking I did seemed to show that folks liked the Limbsavers more than the Pach's. There was some dispute about fit with the 500's. Make sure you get a prefit that's intended for a synthetic stock, not a wood one, for the 500 series.
I can't say that the ports on our 835 make it any louder. I do think a shotgun barrel any shorter than 24 inches seems louder, as it's closer my ears!
The alloy receivered Mossbergs have a reputation as kickers with 3" and 3.5" shells. But standard velocity 2.75's are milder for most folks. I think the porting and hi-tech pad will make your 500 very shootable.