Noting that it seems "Fast Rasterizer" is generally a speed improvement for a lot of Mental Ray render scenes, it does not work with Forest Pack Pro, even with some adjustments to the settings and Forest Material. Of course you are warned when trying to render with settings other then those recommended by Forest. Still, a little tweaking and testing with different settings or non-conventional workflows can often lead to a better understanding of the conventional process. So disabling "Scanline" and hence the Fast Rasterizer is required for the Forest Pack trees to render correctly with the Forest Material.
Although replacing the Forest Material with a conventional 'Standard" Material works, the render times do increase. Obviously the Forest Material shader is optimized by itoo for Mental Ray and works very well.
In addition, testing with Mental Ray Proxies vs the Forest Pack's Planes (using the same forest but the proxies as 'Custom Objects") shows that the Planes render faster. Surprise there.
Obviously Forest still suffers from the dreaded Top view scenario, but I have a few ideas for workarounds that I'll be testing.