That's really impressive.
So I had a play with your .tgd file to see how it worked (1st image), and thought, why not replace the Power Fractal with a Density Fractal with the same settings apart from the displacement (to my logic it's just a black and white image), maybe that would speed up the render. The result is the second image. Then I thought, surely the original Density Fractal controls the hight, so turned off the Y input (3rd image).
Hrm. Nothings changing, so I directed the original Density Fractal into the Cumulus Layer, skipping out the redirect node (4th Image). Again no change.
I set these preview renders with 1/1 GI, as this is just for bouncing light rays around, shouldn't have much effect on the cloud structure, so I thought. Is this the case, or am I doing something more fundamentaly wrong?
All render times were pretty much the same @6mins on my machine.
I really like the idea of distorting the X,Z along Y. Especially if I have a specific cloud I want to copy. Or am I again not grasping the subtle changes these nodes make?