Hi Johannes,
The glow of the fog can be set using the Intensity parameter. And you can darken the profected shadow of the cloud on the scene by increasing its density.
Currently the fog can't receive shadows from other objects, as well as it can't receive light from ligth cones and light bulbs. These are 2 limitations I'm aware of, but I couldn't find a way to solve them with Carrara 3 or 4 architecture. I'll complete the volumetric model as soon as a solution is available.
About the evaluation version, if you lower the renderer quality ( setting the Antialiasing to Fast or None, and the Pixel accuracy to 2 or 4 pixels ) you should be able to render more than 4 or 5 times the primitives. Let me know if you think it's still not enough with these settings, I can build another demo that authorized a little bit more renders.
Julien