Well after messing about I can definitely say that it runs faster without collision enabled on the trees. I actually ran out of memory before I ran out of enough speed. Fair enough it wasn't super smooth and did some occasional pausing to load chunks, but I did put down a LOT of trees. I'm sure there is a bug causing some issues there myself.
Also, I managed to do a level with grass covering a huge area, spanning across the length of the entire terrain, and it still ran 40+ fps np. In fact it was mainly 70+ but occasionally dropped down a little.
The start of this video is the scene I messed about with set at sunset. I added a few more bits to spice it up and played about with the settings to get a nice balance of graphics and performance. I also have other scenes in there, that you may or may not have seen.
Still, grass at least on my system, is without doubt working way better than it originally did! I'm pretty sure you could paint the entire terrain and still run about no problem. Now we just need a similar system for trees