Why are my trees having 14,500+ batches?

I'm using primarily detail (trees) on a set of 16 smaller unity terrains, with a high quality (read high triangle count) set of trees (about 20 differing trees), with a total tree count of roughly 60 (big) trees.

Trouble is, as the screenshot shows:


I have a load of batches and a lot o draw calls, resulting in useless framerate. Remote the trees/details objects on the terrains (Terrain Options> Tree & Details> Draw (Disable) and the frame rate is 60+fps.

Having scoured the forums and websites, I come to the conclusion that trees drawn through Unity Terrains are not optimizable....

But I see stuff like this asset on the assetstore, which promises to optimize even the trees drawn by Unity Terrains...

So, how can I optimize these trees? GPU Instancing on the shaders/materials/Terrain Options does nothing... I am familiar how to bake meshes together, create all in one materials, but the trees produced by the terrain respond to non of this, as prefabs/indiviual models/objects in a scene do....

Help!