Google "Netkas eGPU", they can help you get it working.
... Because Apple missed the boat with Intel's "HEDT" platform - ie Intel's' 'prosumer' high end 6, 8, and now 10 core OVERCLOCKABLE cpus that they started producing in 2011 (X79 now X99 platform).
Just as Apple killed off a fully customisable Mac Pro. Own goal, anyone?
If you're an enthusiast/system builder you can build a much better system than the $10K (aud), and have the best of both worlds - fast single core speed, and great multicore. Oh, and any mix of gpu's you could want.
Don't hate on me for saying that - I've been a Mac *consultant* for 25 years. It pains me - DEEPLY -to see Apple on the backfoot in high end content production.
With no 'real' pro model, it's a shame Apple effectively turned their back on the very users that kept them afloat during The Dark Times.
the nMP, is beautifully realised, it just doesn't suit the wider (ie non fcpx) creative world.
FCPX heavily uses a built in - ie baked in to the cpu hardware - technology called "Quick Sync", (google Intel Quick Sync, FCPX) which is a direct hardware call that's been around since Sandy Bridge (circa 2011, i7's etc). Hence the speed on 'low' systems.
Adobe products, that use gpu acceleration, favour "CUDA", which is nVidia's gpu technology.
And yes, getting an nVidia gpu to work on a nMP is technically achievable, but its a bit of a kludge. And... the cost of the Thunderbolt enclosure (with an appropriate psu) is pretty close to a high end motherboard and cpu. You can easily plug 2,3, 4 gpu's into a motherboard - something you bought once. Making 2, 3, 4 external eGPU's - again, its technically doable - but astronomically expensive.
Not "out of the box".
But it is possible with modification of a key "kext" file.
Bizon and TI both 'appropriated' a script written by another, google 'netkas eGPU' and you'll find instructions from the person who originally cracked this, AND has a deep architectural understanding of how to get it to work on your system.
TI looks great on the surface, especially for eGPU noobs.
However, the TI guys, one in particular, is the one who stole it from the originator - A guy by the handle "Netkas" (wrote the script with "Rominator"). Old hands in the HackT scene, and actually developed a key "kext" file (FakeSMC) that makes HT possible.