'Combat' shotgun, unless you've got a rifled barrel and are using slugs, it's relatively useless. Rapid engagement/snapshots on paper targets (silhouette), I've no problems getting accurate hits (COM or head) out to 50 yards, and am acceptable (COM hits) out to 100 - that's with a mossberg 500 presuader with Hogue rubberized stock and just the beadsight that comes on the 18" cylinder-bore barrel, and crappy eyesight.
Spend the money on comfortable furniture and/or (upgrade?) a better shotgun. In terms of modding a shotgun, I think the only modification I MIGHT want is to get a speedloading kit for when I don't have the time to reload manually from a shotshell carrier (pouch, bandoleer, etc.)
Don't get be wrong, a red-dot on a shotty looks REALLY cool, and has a ton of novelty value, but at the distances at which a shotgun is used (well inside of 100 yards), I actually prefer a glow in the dark bead, as to keep my field of view as open as possible (same goes for paintball [which is the closest I've come to shooting targets that shoot back] when playing indoors [shooting inside of 10 yards], but I leave my red-dot on because I need pinpoint accuracy, since I can't just shoot through obstacles and have to hit that one inch of the careless idiot that's sticking out from the side).