Wow, beautiful indeed! Were you using the new Robust adaptive sampling option? If not, you should try it, will probably increase quality/lower render time, especially for path tracing.
WAS, Voxel Scatter Quality is only going to have an effect if these are v3 clouds, which as has been discussed here before is not really necessary for thin mist like this. So if it is v3, rather than reduce AA and up voxel scatter, I'd just switch to v2 clouds, and probably save even more render time with higher lower noise results. Although changing to v2 clouds might take more adjustment time to match this result if it's already v3...
- Oshyan