USB risers are not using USB protocol, they're just using the cable since it's pretty durable and works easily even over distances (unlike ribbon).
GPU bandwith doesn't matter in terms of the riser because the GPU isn't pushing all the calculation data back to the riser and the system, it only sends back what it solved from the huge amount of data.
Which is why 1x PCI-e (USB risers) is more than enough (250 MB/s) for virtually every algo.
Which is why I thought it. I also wondered about adapter cards which simply provide 3-4 pcie x1 slots in a single x16. No-go. Couldn't find any.
I don't know of a USB 3 controller that could process say 10 separate risers.
Who needs it? And it wouldn't work, anyway. A single controller would be bandwidth-limited by 10 cards. It could do five, though. There are PCIe x16 -> 5x USB3.1 cards. So three cards in three slots provide 15 cards (not many mobos with 4+ full-bandwidth PCIe3 x16 slots), plus the onboard USB3 ports, if any. That's three or four separate 6GB-bandwidth controllers.
But I think it could be done with the right thunderbolt connections as a mobo to t wire to pcie does exist.
And t bolt wire can handle a lot of data quickly.
Btw I like the way you think
That's very interesting. Alienware makes a similar unit, with an inbuilt PSU big enough to handle _real_ video cards. ;-)
But, ultimately, we'd want to gut the case because a bigger PSU powering more cards is more energy efficient.
And we still don't actually need the bandwidth of Thunderbolt. USB3 is more than enough and considerably less expensive. I can't find the link, now, but while I was searching around last night I saw a header in the AMD forums suggesting they're working on USB-interface cards specifically for use as math coprocessing, which is what altcoin miners are using them for, but they also are in use in render farms, weather centers and anywhere else in need of more floating-point-operations-per-second (FLOPS) than CPUs can provide.