I have heard people using external graphics cards on thunderbolt 1/2. From what I know it tends to be a bit hacked together. On the hardware side, you have to find a thunderbolt enclosure with its own power supply and those can be expensive. On the software side, OS X seems to take quite a bit of work, since driver support can be spotty. Windows seems to be easier to get to work.
Performance on thunderbolt 2 is pretty good. You tend to get 90-95% of the performance of the card in a classic desktop. The reduced performance is because of bandwidth restrictions, and thunderbolt 3 obviously improves that.