Ive also generally seen refresh rate jumps when sprites are close to the camera, just like the foreground bush you mentioned. It dosent even have to be a million particle effect, and could definitly be recreated with a few sprites. If you can "break" the camera and go into free look, I bet you can get better refresh rates when you bring the camera far away from the whole plant scene yet keeping all the plants in the view. Its the same number of visible plants, yet the refresh rate is better the farther away you are from them. I think the overdraw idea is in effect here too.

Dare I suggest this, because its alot of work but

Maybe you can make extremly low poly models of the plants when they are close to the camera. So that pink flower plant in front of the camera in your screenshots would morph into a low-poly "high detail" model. However this model cant have any transparent areas with overlay, and probobly a smaller skin resolution. Maybe this model would bypass the overdraw problem because it has no transparent areas for the engine to reiterate, and imo is generally much better looking than seeing a flat billboard of a plant that stares at the player when he rotates.

Though be assured this problem is tricky to resolve, and you really cant skimp out and cut corners. Getting a scene to look awsome in a screenshot is easy, but getting the scene to flow in realtime without hiccups in FPS is another monster.