I've got a very similar setup as wrl, except mine's a 20" with the factory 8+1 capacity and the factory ghost ring sights. Though, I do plan to get a meprolight tritium front sight, just the dovetail blade version.
I wanted to thank you wrl for posting the Limbsaver & 870 adapter setup. I had read about it before seeing your pictures but this thread cemented it in my mind. I actually ordered the adapter & pad before I got the gun. It's SUCH a nice setup. I shot magnum slugs & buckshot over the weekend and feel absolutely nothing in my shoulder. When I got the gun originally, I tried a magazine of birdshot with the Magpul pad that comes with the SGA stock, it kicked hard and hurt. I almost regretted getting the gun, I knew I wanted one but it had been quite a while since I last shot a 500. The Magpul pad sucks. Once I got the Limbsaver on though, oh man, it's such a joy to shoot. So comfortable with no harshness. I gladly would have paid double for the Limbsaver and highly recommend it to anyone wanting to shoot their shotgun more.
For anyone contemplating the Magpul furniture, do it. It feels great, ergonomic and well made. One thing that seems to be overlooked too, the Magpul edition of the Mossberg is the only non-military (590A1) version that's parkerized. Most all others are blued. The Magpul editions are really good bang for buck, especially with the sale that's been going on over at GunBuyer.com for a while now.