I'd rather see a less-dense demo that ran at 100 FPS than a crazy-dense demo that runs at 50. Rendering foliage should take a minor part of your rendering time, considering that a real game will probably add another 30,000 polys to the scene. I suggest making a demo with a larger area with less dense foliage that runs at a higher FPS. Of course, Blitz only renders so fast, so I assume your speed will be no better than any other masked single surface mesh.