Thanks for donating your brain power!
The billboards are imported and having just checked them (I had no idea they were being used to be honest), they are being utilized by all the trees via the Billboard material inst.
You make a good point about distances though! I should include some more information regarding that, included are 2 more screenshots of the same scene from different angles.
Running at 36 FPS…
Running at 33 FPS with nothing but clean cut edges in view…
Thanks again for the input!