Yeah your colors have really improved. It feels artistically framed too.I'm not really too sure. I haven't looked into a ton of optimizing. I do know my foliage is the biggest culprit in performance because of overdraw of opacity. Thats where I have been creating my meshes with more tris to eliminate as much overdraw as possible.
How much control does Unreal give with optimizing? What's the draw call count?
Also I think using Simul TrueSky has helped tremendously because it has it's own GI solution so it's really helped.