After some initial testing -
In the 'Type' settings.
*The type of cloud has to be selected before placing the cloud object onto the map, after that it's type can't be changed again.
In the 'Configuration' settings.
*'Ra' is Radius a, which is how far the clouds spread North/South on the map, and is limited to a maximum 100,000 units.
*'Rb' is Radius b, which is how far the clouds spread West/East on the map, and is also limited to a maximum 100,000 units.
*'H' is cloud height, which has a maximum of 10,000 units.
*'Rain' doesn't work.
*'Dynamic' doesn't do anything that I've seen.
In the 'Properties' settings
Edit- I just opened the FMB up again and there are seven more sliders/variables there than before, but I haven't had time to check them out.
*'Thickness' is limited to a maximum of 350 units.
*The two sliders max out to ten but I'm not sure what they do, or even what they're called, it may be a subtle effect related to the clouds sense of solidity and movement.
In general, the advanced clouds aren't an absolute frame-rate killer under all circumstances, and they can actually look pretty good. But I'm not seeing anything much better than what we had in IL-2:1946, and what does look different doesn't seem to be working properly as far as I can tell. Might be good for screenshots or very selective video shots, but a lot of it just doesn't look very good or is outright faulty. That's what we were told before release though, so no surprises there.
In testing with a single Hurricane in the middle of the Channel I've been getting varied but consistent frame rates from in the teens to into the forties with all video settings on full. Would have to look further into though to see if the extra clouds really are playable, some definitely aren't.
In terms of bugs, there's an immersion-killing sort of pop-up/drop-out happening with the clouds when maxed out to a 100,000 unit radius. It's not at the edge of the radius itself, but their viewable distance. Which means they don't provide a solid overcast as far as you can see.
Putting the 'wrong' settings in will cause a crash to desktop when the mission loads.
If anyone can add to this or correct any mistakes in what I've found, please do.
Edit - Here's a short vid showing something that looks like overcast when flying under some low level cloud. Saw some other nice looking clouds while looking into this, but would have to play with it further to show something good that no-one's seen before.
Edit2 - Just for reference, here are some more examples of cloud effects using the FMB -