As an update, thanks to the great work of Nando with his eGPU software, I was able to put together an eGPU setup soon after this thread
It is comprised of a 550w Corsair PSU, Sonnet Expresscard to Lightning Bolt adapter, and an Nvidia 650 Ti. I've been happily gaming on my MBP pro for over a year now, including a lot of new titles (DX11, 1920x1080)
I'm using a Sonnet III-D and a GTX 760 Ti, and it runs pretty good on my 15" rMBP