It could if you race and every % edge helps. I don't know if you'll be able to tell the difference personally tooling around the lake or ocean. The aftermarket a bit expensive for what you get. Looks cool though and I'd get them if I had $200 laying around and a crew to show them off too.
The sponsons are for your turning. The lower it is the harder you can corner, the higher up the less traction (so to say) you have and the butt end slides out. They would have no effect in how stable the ski is with more people on it, especially if the ski is stopped.