The benefit of the dual motor mod is torque. Yes, if you dual cell a dual motor bit, you essentially have two single cell bits driving one chassis. This will obviously give you more torque due to two motors/vs one chassis, but I'm not sure how much improvement you would get for top speed. As I stated before, a dual celled/single motor bit is pretty much as fast as you can go (without constant flipping) for racing on a track.

As far as your other 3 mods, you can find a lot of info using the search engine.