Also with mental ray you are locked to I believe 8 threads or cores not sure which. You have to run standalone to use more CPU cores. It would also be nice if you could use GPU cores but most professional systems do not use SLI but instead ship the rendering off to a render farm, which of course is cpu based. So I doubt we will see gpu or cuda enabled maya any time soon.

How long is the animation and what is the ave time to render a single frame? You clearly have a bottle neck and it could be the memory to disk. Once the system memory is full everything is then throttled my memory to disk write speed.