I just got my eGPU set up running on my new 2015 13" Macbook Pro. Essentially still at a fresh OS X install. I guess I'll do a thread once I get some of the kinks worked out. My GPU is an EVGA GTX970 SC ACX2.0, also using the Akitio, DA2, and same cable from ebay. Here's a quick run down of what I did, mostly what I did differently (I didn't do any OS X set up, really just want to use it in Windows):
1. Installed Windows 8.1 via normal bootcamp process on internal SSD partition.
2. Updated bootcamp drivers, with intel graphics drivers (didn't think about this when it happened...)
3. Installed Nvidia drivers - custom: only the driver and physx.
4. Tried to boot a good 10-15 times before it finally worked. Had some boot hang ups, a couple windows errors, but I just kept going at it trying to power on the eGPU at different times.
5. FINALLY it booted all the way into windows.
6. Success!
I've only booted 2 more times and I use the same method. eGPU on, wait 10 seconds or so, turn on Macbook. I'm 2 for 2 so far
I've only tried Half-Life 2 so far but it worked great so I'm pretty happy. I'm not sure how to get it to run on the internal display if someone can help me out there in anyway? I've never messed with nvidia experience so maybe I need to mess with or activate Optimus there? I did reinstall the drivers with nvidia experience and everything to see if it would magically work. I know I still have a lot of tweaking to do, underclocking etc.
Not quite the answer you're looking for but it may be potentially applicable if there are coil whine reports around the internet for the mini 970. My EVGA 970 has coil whine in my desktop but not so much that I can hear it unless I get down next to my case. In the eGPU setup, there's definitely coil whine and can be quite loud... but of course Half-Life 2 is running at like 300fps and the higher the fps the louder the whine.