I was looking at that kind of thing as well and the only thing that's worth noting is that I think a lot of video cards currently actively used are only 16x so they can be run on older motherboards with pcie 1 or 2.

It's not clear that thunderbolt 2 isn't enough bandwidth to effectively run a video card, though I think you'd probably run into problems daisy chaining video cards with video cards or trying to run two out of one enclosure.