I think you may have misunderstood. I was saying that you should *not* have changed that setting, that it should remain set to "None (Highest Detail)". If that's still where it's at for all of your cloud layers then the culprit must be something else. I'll see if I can think of other possibilities, but seeing your TGD would help.
Alex, the reflection shader doesn't handle softness that well across bucket boundaries. So if you have any softness, it requires very high numbers of samples to resolve well. The softness values are also extremely sensitive, you want to use very low point values in general.
- Oshyan