I have reported it to Laminar, who I'm happy to say responded very promptly to say it is already noted as an issue/bug. It is not currently supported in MacOS version. But they know about the issue and so hopefully at some point things will improve.
However, I have had success using the set-up in Bootcamp (windows) on the MacBook. Your license will work for the windows version too.The eGPU outperforms the onboard here. I was put off trying bootcamp due to all the complicated instructions on egpu.io, but it seems that on this MacBook (purchased January 18) everything was simply plug and play - no fiddling! But be warned - others with different set-ups don't have such an easy time (https://egpu.io/forums/implementation-guides/mid-2017-15-macbook-pro-rp560-gtx1080ti32gbps-tb3-sonnet-breakaway-550-win10-tony-blanco/#post-31078
On High Sierra 10.13.3 (no betas).
A fresh bootcamp install, fully updated.
Boot into windows and plugin in the eGPU
Let windows install drivers. Reboot etc as prompted.
I then used device manager to update drivers, just to make sure have latest
Now in X-plane, I can have HDR enabled, and can even run with max objects, though one notch down gets me ~30 fps reliably. In some places on the map it pushes 40fps. I'm even tempted to try and get a Vega 56/64 to see how much more I can get out of it. Though it is clear that the cpu is also a limit at various points. Not tried nvidia as I have stuck with Radeon for now since Apple tends to be supporting natively.
But honestly, it works in Windows Bootcamp. Try it!
I will just have to be patient for it to be better in MacOS too and for now have to put up with rebooting into windows.