Apple is supporting external GPUs with High Sierra // US$599 Apple Dev Kit
Improvements yes. With the GM + HL23T's macOS TB3 firmware & RX VEGA 64 , I was able launch Hitman Metal API benchmark and it went through almost to the end. The app crashed immediately in my previous test. I don’t know if it’s due to firmware or GM build. Probably the latter.
Quite a lot of games already support Metal:
I can confirm that flashing my XFX RX480 XXX OC Edition with AMD's reference RX480 bios works perfectly. I have acceleration under MacOS and Windows 10 using the Akitio Thunder 2. Do make sure you use a bios with the same VRAM manufacturer, for me it was Samsung.
How do you flashed you graphic card ? There is a link or video ?