Works pretty well, and my latest addition is a DisplayPort switch to use the iGPU directly with the monitor (when I'm in macOS) or the external GPU (when I'm in Bootcamp). Performance is great, around ~95% of desktop GPU I'd say.
The only issue is that it needs a particular boot sequence to enable it, when switching from macOS to Windows with the GPU enabled: reboot from macOS, turn on the GPU, the default partition is Windows so Windows will boot with the GPU enabled.
To reboot from Windows to macOS, disabling the GPU: reboot from Windows, hold Option at boot to show the boot menu, then turn off the GPU and boot into macOS.
Not that bad once you get used to it, and having a desktop GPU on my laptop makes it all worth it.
My post on /r/eGPU: https://www.reddit.com/r/eGPU/comments/77e86g/success_macboo...