[Darryl Ramm]

Look it is simple. With that switched adapter card you need twice the PCIe 2 lanes as in your Mac Pro as PCIe 3 lanes on the M.2 cards. One M.2 card you need 8x PCIe 2 lanes, two M.2 cards you need 16x PCIe 2 lanes. Talking electrical lanes of course, not just slot widths.

IIRC HDN is 1 lane x PCIe 1 electrical in a PCIe 4 physical card. You could not find a more wimpy card spec if you tried...