I think I understand - and I did such a test on the Disney thread - adding a very small bump map to simulate the microsurface (see below). This would be an ideal scenario - since all you would need to do to simulate any surface is plug an appropriate bump or normal map into a perfectly glossy shader.
The problem is - Blender doesn’t handle textures with a very small scale very well at all and you can start to get all sorts of image artifacts if you scale the texture down too much.
One thing the test did show however is that (in spite of the image artifacts) simulating ‘physically’ it in this way does give closer results to reality and to my node group - than just using Fresnel, so I guess it kinda validates the concept.
