Great questions and remarks, Martin. I too like the upper half most, but mainly because I simply used my "default" procedural terrain as a placeholder :-)
I agree the blendshader for the scattered cirrus is waayyy to visible, but I've improved that for the next revision. I will do the terrain in version 3 or 4, depending on the progress I make with the sky.
The image came out of TG2 pretty good already, but has been postworked nonetheless. Essentially I made 3 adjustments:
1 - the cirrus shone through the cumulus in a few places, so I copy-stamped them away
2 - the overcast part in the front cloud was very strong, so I mixed that part with a less exposed version of the same exr
3 - I carefully ran the image through Picasa's excellent "glow" filter, which works awesome with skies of all sorts. There you go, I gave away my best-kept secret. I regret it already ;-)
Cheers,
Frank