Sadly, the blocker for external graphics isn't the power draw, it's that Intel steadfastly refuses to license Thunderbolt for external GPU enclosures. That's why you can't buy a macbook air and plug in a little $300 box with a GeForce 970 and play high-end games on it right now. There are no technical reasons why this won't work-- in fact, people have hacked together solutions that work great. Intel doesn't want to l…
I suspect this is because they think thunderbolt 1 and perhaps even 2 do not have the bandwidth to give satisfying graphics performance. This might change with thunderbolt 3.
[0] http://www.anandtech.com/show/7987/running-an-nvidia-gtx-780...
[1] http://forum.techinferno.com/implementation-guides-apple/427...