have you tested importons and fg together? should be faster then fg alone, could reduce the rendertime a bit more.
Well, I tried it with the default settings and it took 10 times longer to do the importon pass than to render the entire image before, and then the render took 3 times longer.
Obviously, there's a lot I don't know about importons and how to use them but doesn't look to promising. MR is not known for it's intelligent default settings and this is no exception.
Documentation on it is light to say the least. Basically, how to turn it on, that's it. It seems like their tech writer has quit altogether or something. I'll have to find more on it somewhere else. I guess the days of getting real manuals with your software are gone gone gone.