Thank you for your interest in my work.
If by "heavy" you mean the load SkyControl puts on the cpu/gpu, yes, there are many ways to reduce that. TestSkyControl exists to show off all the features of the software, so disabling features you don't need (like bloom or night sky or multiple cloud layers) is a good place to start. Perhaps for your application it's not necessary to update the sky materials on every frame. (I imagine most apps don't need sky updates at all. In that case, one could simply apply SkyMaterial to a DomeMesh and never modify it.)
The opacity of each cloud layer can be adjusted independently. That's one feature of SkyControl that TestSkyControl doesn't demonstrate.
I'd love to make the clouds darken realistically in response to the direction of the main light. However I don't see any practical way to do this. If you have ideas, I'd like to discuss them.
Most of the textures are procedurally generated and can be reused freely. The t0neg0d textures come with a BSD license; I can't help you there. The moon textures and the cyclone texture are CC-By-SA; if you want to distribute them under a different license, I'm open to discussing it.