T'bolt supports PCI, but not all lanes of the data. Theoretically it "could", but to what degree, that I have never seen actually tested.
The other issue is, it would probably be recognized as a secondary GPU, not the primary one your main monitor (MBP screen) is running. Thus FCP X and Motion 5 may not utilize it at all. Again, never seen anyone test this out.
and you will find many more doing a google search. Most of these people seem to do this for gaming on a slow or older Mac.
But let's be realistic.
If this is for editing you will need a decent external graphics card, which will cost you between 500 and whatnot dollars. A good PCIe box will cost you another 500 to 900 dollars. So this external setup will set you back at least 1200 dollars, and I won't even add the time and the trouble hacking the card, and the headaches to make all this work This will give you an external GPU that will run somewhat better than your internal one, but still not as fast as it really should.
If you have at least 8 GB RAM installed on your MBP, you are not editing from your system drive and you still think your machine is too slow for your FCPX work I think it is far more realistic to save a little, then sell your 2012 MBP and get a new Mac with a great GPU that runs at its full capacity. It will cost a little more, but it will certainly run much better than all these hacks. Just my opinion.
Those hacks work for games, yes, but FCPX and Motion will never recognize the external (secondary) GPU. You'll gain nothing. FCPX and Motion will only recognize the primary GPU (build in). Only with dual identical GPs can you gain anything.
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