Mental ray's default AO shader is horrendously innefficient, I would strongly recommend using another method. The final-gather based "fast" AO is a lot quicker, but you loose a lot of control.
A trick you may consider is rendering the AO at a lower res and up-scaling it; AO is usually smooth enough for this to have a negligable impact final quality. You could probably go as low as 1024 and still get a good result, in 1/16th the time.
You might also try Blender for the AO, in my experience you can get results just as good in a fraction of the time. Combine this with using a lower-res for the AO and it should only take a few minutes.