> Luckily, changing the color profile (in system preferences, displays) to "Generic RGB" or similar disables this, and it gets the ~800MPix/sec level of performance similar to the RMBP, which is at least tolerable.

Did author try to send image with 64 bits per pixel? It should by send to GPU without converting. But If the fullscreen is 15 megapixels, than it's ~120MB per frame.