Thanks Marco.

Here's what I've figured out so far>

You can just about retain perfect voxel resolution with a DECIMATE of 70-85%. The problem is you need to do this for each and every layer (shortcut key!) In the picture below are 3 of the same models at various levels of polygon reduction.

The closest part (far left) is the original 4.1M poly model. Notice the little parting line on the side of the top.
The next part is the model with a 70% DECIMATION equaling 1.2M polys.
The next part is the model with an 85% DECIMATION equaling 620K polys.
The last part is if you just EXPORT as SCENE and try and reduce polys then. It is set at 80% and is 831K polys.

As one can see, there's little change on the three on the left. The EXPORT as SCENE option with reduce polys is no good. So, each and every layer should be manually decimated somewhere between 70-85% (as they suggest).

This enables exports that can then be reimported and PBR mapped, then exported again.

Here's the model tests (rendered in KeyShot-- click to view full size)



And here's a quick proof of concept where I created a model, decimated it, then added PBR maps in 3D Coat and exported to KeyShot!