flmason, I have had depts. buy 870s right off the rack, bring them to me, I would cut the barrels down, re-bead them, and put them in patrol cars. We now live in an era of "custom combat guns" where you have to have this, or you must have that.
I did put some different springs in my personal 870, and my personal 1100, but was it necessary? Most likely not. I also changed the stock on my 870 as I prefer the old corn cobb forearm and the more sleek butt stock of a fighting shotgun. As far as heavier internal parts on either the Mossberg 500, or the standard 870 Remington, you don't need them, they will shoot anything their chambered for. Gunsmiths would like you to THINK YOU DO, but their in it to make money.
The hardest thing on either is to break them in. The more you shoot them and work the slide, the slicker they get. I've seen dept shotguns that had been carried and shot so long that if you pointed them straight up and pulled the trigger, they were so slick they would slide open themselves.