Micropoly detail doesn't matter if you're not rendering the terrain *and* you're using Defer Atmo (which you should be).
AA of 6 could be fine, the the real key is the adaptive sampling and the Noise Threshold.
For GI in clouds, did you test with lower settings? Unless you are animating, you may not need it that high.
The other thing you should look at is the Voxel Scattering Quality in the GI in Clouds tab of the GI Settings window (accessed from Quality tab of Render node). Default value should now be 100, though yours may be at 50 or something else. Try 100, and try higher as well, depending on your AA adaptivity settings, and cloud quality settings. You may find a value of 100 here, with lower cloud quality (e.g. 0.5) works better, for example.
There are a lot of settings to test, so ideally we can just see the TGD and make more direct suggestions based on actual tests.
- Oshyan