Hey man, I've got a 2012 Mac Mini connected to a ViDock 4 and a nvidia 670 GPU inside. I've got a dual monitor setup (one 27inch 1080p, one small 15inch screen). The GPU works in both Windows 8 and OS X Mavericks. In Windows, I use it to play games like Battlefield 4, Titanfall, DayZ, etc. and it works great. I play at a constant 60fps with a mix of medium and high settings at 1080p in bf4.
For me, I already had a Mac Mini and didn't want to buy a completely separate system just for gaming/video editing. Now, I've got an extremely capable gaming system thats about twice the length and width of the Mac Mini (pretty damn small if you ask me).
It does take a while to setup, but works like a charm once you get it working. For Windows, theres a boot sequence you have to do on startup (pretty much unplug, then plug back in) and for OS X you just leave everything as is and it boots up fine). I've also got a HP 4530s that I hackintoshed and the eGPU works fine with that system too.
And I'll be the first to tell you that most of the people who bash on external graphics have never used them/dont really understand them. I've been using external GPU enclosures with several systems and GPUs for over two years now so if you have any questions, feel free to ask
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While basically possible and in theory also able to achieve decent performance (though slower than GPUs hooked to internal buses), the whole thing won't work due to one thing: driver support under OS X.
Last thing I heard was that it's not possible to use an external GPU attached to a Thunderbolt port under OS X since the system simply does not support that.
Under Windows on the other hand, you could have success with that. The few examples I know where someone managed to get an external GPU to work via Thunderbolt all stressed they had to use Windows for that.
Works flawlessly in 10.9 Mavericks. The tutorial actually makes it seem more complicated than it is, it takes about 5 minutes to copy/paste text into three files. The hardest part is finding the files lol.