Performance improvement is always good. Raising limits is also always nice, though depending on context, it might end up not being used much. Object limit would definitely be of interest to gatling gun fetishists like me.
Never enuff dakka, as they say, and realistic gatlings do provide quite a lot, at relatively little cost (they don't usually need to many flashy/hi-res effects). Mods like Diaspora or Dimensional Eclipse could probably use this.
It'd be a nice to see the asteroid system get a serious rework. The current implementation is very limited. For example, you can't have multiple fields in a mission, or multiple "inner boxes" (areas where asteroids don't appear). Also, as mentioned, they're quite a drag on performance. If you're willing to tackle that part of code, you can improve a lot.