Thanks for chiming in, Matt. I indeed found that with smaller roughness, render time increases. I will set up another test to see what I actually did, and post it.
No, the 'real' reflections are by reflective shader, masked by a reduced PF mask.
I made it blue to have a blue-sky-like reflection, multiplied by a distance shader, so it's bluer (more blue) near horizon. But that was before I added the clouds, so in fact it doesn't make sense anymore.