I'm running Octane X on my traschcan Mac Pro 2013 with a 5700 XT in a Razor eGPU.
It works just fine in standalone working with materials, setting up scenes, but it can hang on more intensive renders and is prone to kernel panics on long renders. Possibly related to the known MacOs bug with Navi GPUs and connecting to TB2 with the Apple TB3-TB2 adapter? For simpler scenes, it does just fine.
I haven't benchmarked it, but in my personal experience it renders at about the same speed as a 1660 TI running Octane 2020.1.5 under Windows.