I think the key is the size of the grass clump. It must be small enough to create a clean edge, but not too very small or the render time would increase significantly.
If that helps, i've attached a grass sample scene that creates acceptable edges and it renders pretty fast. I use a Forest object to create the grass clump, and other Forest to scatter it. Since both objects are parameteric, you may adjust the clamp radius and the density to get different results interactively.
This scene uses Max 2010, Forest Pro 3.3.6b and VRay 2.0.
PS: The Forest used for the grass clamp must use Display->Render Mode->Meshes, in order to be used as mesh source by other Forest object.