What Acadia said of course applies , if you dont have enough memorty then it starts to page out. You seem to have ample memory. I would try 4 threads only. It is possible to use more threads than processors if each thread is doing large amounts of disk I/O but it can slow down processor heavy applications.

Overclocking would make a big hit but you need to concern with overheating.

If you went from 5-6 min to 3 min thats a fair saving. It would cut a 4 hour render to maybe 2.5 hours.