Ok, I think BB will have some fun figuring out how to shade cloud layers. :D
Your method to get an scene-matching IBL image is cool. For some higher quality picture the additional effort surely pays off but the best part is that no other applications are needed except Poser itself.

Patorak, that's as how I understood it at first.
I never saw this kind of artifacts as Angelouscuitry experienced them if doing a skydome.
The confusing part is 1) how exactly he placed the spheres to get these artifacts and 2) what he wants the reflection to look like.
As BB said, the shading area subdivision, or min. shading rate, would make very high poly skydomes useless.
I think up to 1000 ... 5000 polys, depending on hardware performance, are fine for most cases.
Together with figures, hair and clothes that's a heavy load for an PC.

Angelouscuitry, I forgot to answer that sphere-question. A sphere is only the shape, it can be hollow or not like an billard ball, can have an theoretical wall thickness or a real one like an basketball.
The Poser sphere primitive is hollow and that means you can not tell Poser that you want some attributes (colors, shaders) for the sphere volume. There are no points inside the sphere that could store these attributes.
If you model a massive sphere and import it to Poser, you can assign attributes to the volume and create some sort of (animated) atmosphere for example. For an sky you would still need an additional sphere because of the way the texturing works.
1 sphere for the skydome, 1 sphere inside the first one and a tiny bit smaller that simulates some atmosphere that can be faster calculated than the Poser atmosphere node.
For example useful for fog (moving) over the ground, volumetric clouds, rain, ...

Or simply as a mean to apply filter mathematics to the picture. I.e. if you look through sunglassed you would see a filtered version of what you see normally without the sunglasses.