Yes and yes.
I don't have any meaningful estimate of how much faster it would be to use models as opposed to ships. However, they don't require AI, physics, or anything else of the like, and they also don't take up time to check collisions for anything else. I'd guess there'd be a noticeable gain with large numbers. If you notice, the Shivan Knossos was 250 fighters and the Oscillator was 800 nav buoys - the latter is something that's not even possible with conventional Freespace, much less at 6 FPS.
Image 6, with the Shivan Knossos, is running with MediaVPs enabled, too.