I have seen this mentioned on this forum, but not in much detail. I've been doing some research and just placed an order for some hardware to build one for my Macbook Pro Retina but considering the limitations in upgrading video cards in Macs, the mod could potentially boost any Mac by a lot. The main source of information seems to be here: http://forum.techinferno.com/implementation-guides/
Of course, it's gamers who have done most of the experimentation so I'm still unsure how the performance is going to translate to Octane rendering but it seems worth a try. Part of why I decided to give it a shot is this product: http://forum.techinferno.com/enclosures-adapters/7205-us%24200-akitio-thunder2-pcie-box-16gbps-tb2.htmlhttp://www.amazon.com/Akitio-Thunder2-PCIe-Box/dp/B00LTAUTHE
With this $259 Thunderbolt>PCIe adapter, you can theoretically use any graphics card through a Thunderbolt port. The converter isn't large enough to hold high end cards and it's power supply is limited so you've got to power the card separately. It sounds sketchy at first, but it's fairly simple and there are a lot more success stories than failures, and the failures are minor. Basically, some cards work better than others. There are certain limitations on some of the Macbooks but, from what I've read, they mostly just limit the ability for the graphics card to affect enhanced live graphic abilities on some models and the reasons are pretty obvious and related to Apple firmware limitations which is fine with me because it should only be a setback for gaming purposes, not rendering.
Here's the video that originally got people excited about these things. The chassis used here runs around $1000 but the demand has led to some more affordable devices popping up, like the one I ordered.https://www.youtube.com/watch?v=h_wAxRs0YAE
I'm curious if anyone here has used an eGPU with Octane and what the results were. Also, it just seems inevitable that there would be a conversation here about this sooner or later. I could see the trend bringing a lot of attention to Octane as renderer of choice.