I knew for sure that it would be impossible to do with B350 boards, but I have yet to see the IOMMU readout showing the 16x PCIe broken up into 2 8x slots to be able to pass them through. If you have a link to show this, I would be really interested in seeing it. I am also hopeful that USB will work as you describe, otherwise my dream of the ultimate machine, will be crushed.