That is rather incredible.
Randomizing the cloud size more would greatly improve the realism I think, although one of the biggest problems with an overall procedural technique like this is really being able to predict what will appear and where. What I mean is, when I'm taking a photo or composing a painting I look for specific shapes that will lead the eye around the image. In this case I might be looking for cloud formations that nicely "frame" key features in the fore or mid ground, or which particularly add a sense of depth by leading the eye into the distance.
This is difficult with procedurals, unless you can define specific areas in which the effect will take place.
In Vue (when I had it running in V5) I could place specific cloud objects (cuboids whose final rendered formations would be procedural) where I needed them, and had more random and lighter higher level cloud formations as a general backdrop to these. Might be a direction worth considering.

I'm very impressed though - yes, render time(?)

 

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