Why does the Finder run at such a low process priority?
I'm not sure anyone short of Apple's own developers would be able to answer this question, Finder's inner secrets are their domain. If you do think there are some noticeable lag problems with the UI you might want to submit a bug report to Apple about it.

Judging from the sound of things (processes intentionally taking up all resources) this is a bit of an edge case for memory management. I've yet to see a system perform at their best without somewhere around 10-20% resources free, as a buffering aid.

Just curious, what are these processes you're running?