Did a few test, your right, the tree system seems to be optimized compared to crysis1.
First off the physics enable only if you enter the obstruction proxy. Other wise you can't even shoot the leafs from a distance.
Second once the physics are enabled, it's active for 5 seconds and then shuts down.
(unless you keep stimulating the system by firing (and hitting the rope system) or continuously moving through the leafs)
This is not bad for touch bending, probably saves unnecessary calculations, but for shooting the leafs you need to create a obstruction proxy at least the size of the first LOD radius, or you wont be able to activate physics and experience bending from shooting the leafs.
As for the 5 sec delay before physic's turn off, not bad for performance, But there should be a way to customize the duration of the delay? Or your right, it could be a bug.
Hopefully we can get some CryAdmin on this asap. And if it can't be figured after that, submit it under feedback section. they should fix it next update.