Here it is...
Notice the slight slight slightly more noisy atmosphere, but especially notice the quite different looking clouds!
Rendering it now with RTA and 2 atmo samples + 8 cloud samples...see what it takes to make it look as good as RTA OFF
When it comes to practical examples I think it can't get any closer than this.
It's a difficult lighting situation, requiring good sample settings and it has objects which justify using high AA levels and thus even more carefully chosen sample levels for atmosphere/clouds. As this test shows AA8 doesn't offer extreme reduction in samples as it is already slower and inferior in regards of cloud-rendering.
This fits with the previous example which showed atmosphere rendering comes cheap and clouds don't...