I did an experiment with adding back in the base terrain map (spans the entire terrain). Loading it in 4k x 4k size took a big bite off of the fps, but if I loaded it in 2k x 2k, it didn't. I think it brought out some more detail and variation in the look of the terrain.

I also added back in shadowing, and finally managed to place the light such that all the trees cast shadows. I also exported a lightmap for the terrain in L3DT and applied it to the scene with little to no cost.