Chipp Walters said:

"One serious caveat for 3D Coat. You cannot UV map a voxel model. That means you can't use the great PBR shaders and UV map tools unless you RETOPO the voxel model-- which for a model like the one above is quite a problem. It has 8 million polys and no UV mapper I know of can auto map it-- including 3D Coat's own auto mapping routines. The same is not true for ZBrush with regard to KeyShot-- as they have a special KeyShot plugin so there can convert the high density models with PBR maps intact. This is a major PLUS for ZBrush. Plus smart PBR shaders can help hide surface defects of topology modelers. All that said, I really don't know how well any of this works in ZBrush and would be interested to learn more."

How about this: While in Voxel mode, export your high-res model under "export object" then start with an empty scene and import your high-res model with "import per pixel painting", which then creates an UV Map with the size of your choice and you can use then your PBR shaders. This should give you the results you are after, imho.

Regards
Stefan