Just one idea:
Do your grass models have an alpha test material, i.e. not blending? Vegetation scenes are oftentimes slow because many overlapping polygons result in a lot of additional alpha blending which results in a much higher amount of pixels to be drawn. If you use an alpha test material, the edges won't be as nice but the z-buffer can be used and the scene will be rendered much faster.
I don't really know Shade-C but could it be that the grass models are then rendered with regular alpha blending and thus you get a lower framerate?