For anyone who has used an over and under or a side by side shotgun, you know how quick it is to swing the shotgun to your target. This is due to the low amount of weight up front. The Benelli M4 isn't too heavy, but a lighter weight shotgun is quicker.
So any chance you get to cut the weight down from your weapon is generally a good thing. If you must add an optic and/or light, pay careful attention to the weight. You're better off putting additional shells on your body rather than on the weapon. Figure each additional shell adds about 2 ounces to your weapon.
It sounds like the proposed rail from Sidearmor is what you might be interested in. I know I plan on selling my rail and moving to it when it is released.
I wish Freedom Fighter Tactical would sell a forearm replacement that simply had a small place for a picatinny rail section.