I've seen some of his paintings in the MH The Young Museum (it was I believe) in San Francisco, a couple of weeks ago. Also in London and Paris they have some if I recall correctly. Beautiful paintings mostly.
I think you reproduced the atmosphere and feel pretty well here.
I'm not so sure about the curvature and thickness of the waterfalls though. They are bit too curved and quite thin, I *think*.
For a nice isolated area of direct lighting I sometimes cut a hole in the cloud-layer with a distance shader and move that around until I get the directly exposed area in the right position. Quite cool to do, because it offers a lot of space to play with shadows and light. Obviously you knew this already and perhaps you did that here
