Hi James,
yes, my first Mac was from 1993, and I fell in love with Octane with a Mac Pro 2009 with an Nvidia GT120 512MB in 2010, so I have followed all the Mac Octane development from the beginning. I have installed in my Mac Pro during time something like 6x Fermi, 4x Kepler, 2x Maxwell, 4x Pascal different Nvidia GPUs, and it is still running
Now in 2020, we have finally a native Octane version for Mac with AMD/Intel support (no need to waist time with Nvidia/CUDA drivers), and already deigned to work with Apple Silicon GPUs, so roughly any relatively recent Mac will be able to run OctaneX natively with Mac Os 11 Big Sur new Metal driver
About Mac Pro 2013, I doubt that FirePro DX00 series will be supported in future, and if supported, the performance cannot be similar to a GPU of seven years later, I guess
So you are forced to use Thunderbolt 2 connection with eGPUs, slower than TB3 in loading the scene to GPUs.
Please also note that in OctaneX, the AI Denoiser is performed via CPU, not GPU, but you should need a CPU from 2015 or newer for performing AI Denoiser with OctaneX.
Older CPUs do not have the necessary instructions for AI Denoiser.
In my opinion, better to have an iMac or MacBook Pro from 2016 or newer, with Thunderbolt 3, and AI DEnoiser supported, than a Mac Pro from 2013
Happy Mac GPU Rendering,
ciao Beppe