Not without overloading the game. In fact, this collision bug you're talking about has to do wit asteroid density that's too high already, so increasing that isn't the best idea. If the belt is huge, then you will have problems, period. To make the belt seem denser, either center the mission within its confines so that you should never exit it and its size would not be apparent, or make it large but very shallow if it is intended to be a background. The collision bug may also have to do with computer specs, if you're running an older system, but then you risk the same thing on other people's PCs. This bug isn't a code thing, it happens when the system can not fully process all of the information (too many objects on screen)
And why do you consider the Sol belt that dense? It should not be as dense as, say, an asteroid belt out of Star Wars. Collisions in our asteroid belt are not commonplace, and it really isn't that thick. Imagine a roughly earth-sized body, broken up, forming a belt around the sun. (Not that I believe in the former tenth planet idea, but the amount of material is about the same) The area covered by the belt is so large that the dispersion of the chunks would be fairly large, and each piece would be rather far apart.
