Mac External GPU: Experiences?

Ostadan

I am using a late-2013 27" iMac with an NVidia 780M (4GB) GPU.  It's not quite enough for larger Iray renders, and I am considering how to address this.  One possibility that I've been mulling over for a year is to get a Windows system purely for running Iray renders, but that's pretty expensive and not really within my area of expertise.  Another possiblity is an external GPU device like the bizonbox 2 which would be more compact and somewhat cheaper.

Do any Mac users, preferably with an iMac similar to mine (i.e., Thunderbolt, not Thunderbolt 3), have experience with such devices?