Hi,
We know that Mental Ray is very slow using opacity maps (because the way that a raytrace works), but i have not seen differences using Forest objects or billboards.
I just done a test scene, first rendering a Forest object and converting it to an editable mesh, but the render time was identical. If you have other test scene which show different results, please send it to us for checking.
Unfortunately, the documentation of Mental Ray for developers is very sparse, so we have not much information about the MR engine for optimizing it with Forest and opacity maps.