I've been running an SLP Mark 1 (the 22" model) in weekly practical shotgun matches for about a year and a half, using mostly light birdshot loads with just the occasional buckshot or slug. So far I've put about 6,000 rounds through the gun, all in fast, hard competition, all with just the light piston installed.
Mostly the gun has been dead-nuts reliable, just outstanding, and overall I can't recommend it too highly. I like it better than the 1100s I ran for years, and way better than any of the Benellis I've tried. Mostly it's a matter of great factory sights for both speed and precision, and LOW recoil.
That said, it has given me some trouble. Its gas piston issues aren't just an internet myth, because mine failed at about round 2500, the spring inside the cylinder simply shearing, breaking in two (FN replaced it under warranty). Also, I've learned from frustrating experience that once in a blue moon you need to completely strip the recoil spring assembly and swab the gunk out of there, or you'll have weird, on-again/off-again feeding problems. Right now my gun is running great again after a spell of that crap. People who've been down this road before me say that the thing to do is replace the factory gas piston with one "sealed" by an outfit called SRM Performance, and replace the entire recoil spring assembly with a Sure Cycle unit from the same company.
...and yes, with very careful use of a Dremel you can adapt pistol grip stocks made for the 1100 to fit the SLP. I've done it with a Choate stock, and it worked fine. Personally, though, I've gone back to the original factory stock -- balances better, points much better. You can find instructions on fitting the stock on the Brian Enos forum shotgun technical section. I had the link, but sorry, I've trashed it, so you'll have to search for yourself.
Good luck, happy shooting.
