My current opinion on TreeFX... for those that want to add them to your tracks...
While I think TreeFX is a cool and exciting addition to Assetto Corsa, remember to not go overboard when adding them. If you do add them, limit their use--maybe only add them very close to the track, and use the standard trees behind them. Also, provide an option for a user to override them and use the regular tri (or quad) trees. A separate no TreeFX ext_config.ini could be set up as an option? Could TreeFX be managed with a skin?
I am using VR with a pretty good GPU. I've set it up so the render resolution is at the highest setting, while I settled on 72Hz. This produces the best experience for me in VR at this time. I've tested a few tracks that had TreeFX on only portions of the track. When I was on a part of the track without any trees in view, I could easily maintain 72 FPS. However, when I turned to face a cluster of TreeFX trees, my FPS cratered.
Again, TreeFX is a cool advancement, and all the other modeled trackside elements are excellent in most mods... but for me, trees are only in my periphery. A detailed road surface, track edges, and kerb detail are probably the most important thing in a track mod, because that's what I'm looking at most of the time.