I'm noticing something interesting. Projects which would render in about 4 hours, are taking over 9, and they are also filled with noise as if they are PT render, not a SR. I don't remember overcast causing the terrain to be noisy, only the v2 itself. The smoke column is the only new bit to the image, but alone, can render in 20 minutes so not sure it really added that much to the scene. But I'm just wondering if something has changed with how SR renders are processed? I never noticed such grain on terrains with clouds in SR before. When I woke up, it was jugging on the bottom left portion of the blank heightfield terrain (which should render fast) as if it was a PT render and doing the PT slowdown thing...
MPD 0.5 AA 6 Resolution 3440x1440
I've noticed stuff like that as well (some time ago) when adding things like local clouds in some unusual way. I guess there's a slight noise allover that is not properly masked out of the local cloud. Perhaps take everything through a color adjust and see what (negative values) would help?
I've also learned that a color adjust with both black and white values the same (both zero or so) does something to input (that helps in certain circumstances), which I never understood.
I also had a cloud lately that turned out noisy (also in the blue sky), and even with quality 2 and AA6 never became noise-free (and with atmo at 32 samples). Just an ordinary cloud height and density.
For me the new AA sampler sometimes acts weirdly - renders are noisier in general but the sampler needs longer in other areas, leading to a less consistent AA overall with more noise and higher render times.
With overcast clouds, you might try increasing atmosphere samples, too. Wouldn't help with the render times though.