Works, but it does not make you bios handle any more videocards (8-13 depending on bios etc)
https://bitcointalk.org/index.php?topic=2204115.msg33127677#msg33127677 (some testing with how it shifts the PCI addresses.
I have no mining cards to see if those could be added on top (as with the 19 slot Mobos which can take 13 video cards max).

The Acorn Nest X2G could be something else to consider. A lot more money but the PCI switch (there was another 8 USB card for about $100) might give other max number of video cards (but maybe not, it's probably the leftover lanes from CPU that dictates the number).