Considering ThunderBolt 2 isn't quite up to snuff for PCIe enclosures at bidirectional of 10 GBps, USB 3.1 isn't going to be enough just on bandwidth terms.

Not at all. I'm using a GPU via ExpressCard with a single lane. 10 GBit/s is plenty enough. The question is whether it DMAs correctly, and whether it's actively disabled by intel again (as with Thunderbolt).