1. I read that SpaceEngine generates nearly everything on the GPU. Does this mean that on my overclocked graphics card (6550m) I should be getting better performance than I would be getting if it were not overclocked?
Maybe, it depends on whether there is another bottleneck in your system somewhere. The difference in performance probably wouldn't be dramatic. The most important thing is memory.
3. In another thread, it was found that Nvidia cards with low VRAM can use the RAM as VRAM as a fix, and it seems performance improved a lot. Is there a similar workaround for AMD cards?
As far as I know there is as yet no workaround for ATI. I think some users reported that they were able to make this happen, but I don't know any details or how accurate that might be. If you have the latest drivers and it doesn't work, then it doesn't work.