Atmo really has only two cloud layers, a standard cirrus and a v3 layer.
Part of my folly was to have the v3 layer use two seperate density shaders which are combined by a merge shader. I've done that before and the merge always seems to cause slow down. Only used 16 atmo samples and defer atmo was on. The quality of the cloud was 1.3 and voxel buffer very high at 140 mill.
I suspect the biggest problem was Detail 1.0 and AA very high (for experiment mind you, I won't say exactly how high, (but more than 15 and less than 17) don't want to be ridden out of the forum on a photo-realistic rail).
Terrain is a more complex exploration of the conglomerate rock idea I once posted in file sharing. Basically smaller stones are intentionally routed to the surface of the largest stones, but in this case with areas of occurrence varied and non-uniform.