I'm using a Gigabyte RX480 gaming box with a MacBook Pro and have the current X-Plane 11.30b7. There's no change with the beta - the eGPU does not work correctly with the Mac version of X-Plane. This is true in all configurations:
- using an external monitor as the display
- using the internal monitor as the display with Mac Mojave's "Prefer eGPU" checked
- using the internal monitor with a headless ghost adapter in the eGPU
Note it's not that is doesn't work. Mac's Activity Monitor confirms it is using the eGPU at 100%, and some graphic features like SSAO, which do not work with the integrated graphics, work with the eGPU plugged in. It's that the frame rate with the eGPU is 20 fps lower than the integrated Iris 540 graphics. The exception to this is when the plane is over water, in which case the frame rate is 60 fps (versus the integrated graphics 40 fps). In this case, turning the plane so that land is visible quickly tanks fps to single digits (even if autogen is set to minimal).
As others have mentioned, if I run X Plane in Windows Bootcamp with an external monitor, the eGPU works better, giving me a consistent 10 - 15 FPS boost versus integrated graphics. The boost still seems low, though, for an RX480.
I hope Mac eGPU support is added at some point. X-Plane is one of the main reasons I purchased an eGPU.