Once again, stop. A GTX 680 can game on the resolution of the retina MB. The HD7970 can do it as well. Hell, their smaller brothers the 670 and 7950 can also do it as well. You do not need a SLI'd Titan or CrossFire'd 7990.
If physics doesn't allow a good GPU, then don't market it as a "Pro" Retina product. Simply don't.
I would not call 20-35fps "gaming". I call it suffering. I can't once again "stop" as this would be my first instance of "stopping".
What parts would you have included then to make it "pro"? Keeping in mind the tdp limits set for the chassis and indeed all PC's that are >1" thick? Even after that everyone would just start posting how stupid hot it gets and "is it safe?" etc...