I’ve seen an external connected to a under Win8 and OS 10.9.x. It took about a 10% hit in both systems, but still better than the mobile GPU. It was very much a “workaround” solution.
Yup, the Oculus Rift runs at 720. That’s all I need. And your right about Windows. I get a three fold increase in frame rate with the Blue Print demo. In OSX, it’s around 20fps. Windows under Bootcamp is close to 60. I’m running a flashed GTX 570 (2.5 GB) on a 3,1 8 core .