With the sidesaddle, besides the weight from added shells and the saddle itself, overtightening the screws that secure the sidesaddle to the receiver can pinch the receiver, restricting movement of the bolt carrier and potentially causing malfunctions.
Hanging a bunch of stuff off a recoil operated shotgun is generally accepted as a bad idea regardless. YMMV...a friend of mine has a Surefire light and the sidesaddle on his, and it'd choke with light target loads but run okay with full power buckshot. Remove the sidesaddle, and it ran okay. Reinstall the sidesaddle and remove the light, it ran okay. With both the saddle and light, it was questionable. All I have on mine is a magazine extension and a cheap nylon sling, and it runs 100% with anything I can get to fit in the magazine tube.