My first consideration for reducing the recoil of a shotgun is to get it aggressively ported. I have a couple of barrels that are well ported, and it makes more difference to me in recoil than any buttstock add-on I've tried (the Knoxx is a system I haven't experienced yet, but utspoolup's observation rings true). You can get a shotgun barrel ported for cheap, probably less than the knoxx stock you are considering. Not only will it help with recoil, it will also reduce muzzle climb. I have a Mossberg Ultimag with factory ported hunting barrel and it reduces the recoil on a 12 gauge 3 1/2 magnum to about what I feel shooting 2 3/4" skeet and trap loads, in other words, pillow soft.

The beauty of porting a Mossberg 500 barrel is that you can get interchangable barrels for it, without having to alter the rest of the gun.

Find a local gunsmith with a good reputation and they should be able to port it for you no problem.