I would guess that trees are optimal for their environments, which include lots of things that are not relevant to solar panel arrays. For example, trees grow and compete with other trees and plants for sunlight. Trees also have different structural constraints and structure cost/benefit ratios than solar panel arrays. Deciduous trees drop their leaves and hibernate through the winter, replacing them in spring. So I think you're right that nature's design isn't quite optimal for solar panel arrays, but it's a great source of inspiration. :)

After 500 million years nature has tried a lot of designs.

There may be hidden variables or different fitness considerations that apply to nature/trees but not to solar panels.

Not only that, but is there a theoretical peak for that which is necessary for survival, another peak for wide spread population and dispersion, and yet another peak for max efficiency?

Interesting, did they make any progress since then?