a few thoughts on this render:
the cauliflower shapes are great, the trouble is the cloud scenery as a whole. You have this huge blob of very sharp clouds hovering in the front, but cumulonimbus clouds just don't look like this, as even these beasts have a softer base and much less sharp edges in the lower third. with tg2 you can only specify the sharpness for the entire cloud, which is a problem. You need to find a way to cheat, and that lies in either hiding all the lower thrid or half behind a mountain or trees, or place a less sharp and softer cloud extacly where that sharp thing is. The leatter is going to be a lot of re-seeding and experimenting and tweaking until it looks totally integrated with the thunderhead that now sticks out of the softer clouds around it.
Last point: as the way fake scattering works has changed, the shadow sides become problematic as well. You need to work with much higher values for the envro light setting in the cloud layer, otherwise these shadow sides become way too dark - like here.
Hope that helps!
Cheers,
Frank