I'll chime in here as well.
I started with a similar setup (15-inch, Mid 2012 - OSX Sierra 10.12.1), Akitio TB2, evga 750Ti GPU (non-powered). Like you found, the unpowered card ran into a ton of problems. OSX would randomly crash for no reason (5mins - 15mins; did not matter). I hacked the vBIOS of the card to lower the clock but still had problems. I returned that card and got an Asus GTX 750Ti card but the display ports wold not show up properly in OSX (worked great in Win10). I returned *that* card and got this card from Amazon (Gigabyte GV-N75TWF2OC-4GI). I paired it with the Dell DA/2 PSU (custom made power cables).
The new combination worked really well for a while then crashed again running Valley benchmark. It seemed anytime the onboard GPU tried to output on the external monitor bad stuff would happen (jerky screen, kernel panic, etc). Per another thread, I downloaded and installed the "DisableMonitors" Mac App to temporarily disable/enable the onboard Mac screen. This app seems to have fixed the GPU switching issue. So far, rock solid. Valley plays at 30FPS at 2550x1440, my VM does not crash, and all seems good now.
As an aside, I found the Apple Bluetooth mouse to cause some weirdness. Even if the system has been running rock-solid for a while, I get kernel panics if the mouse gets un-paired then re-paired too many times. Seems to only happen with the TB2 setup. I don't use the BT mouse anymore.
If anyone is reading this thread, here is my recommended parts list:
Akitio TB2
Dell DA/2 PSU with appropriate power cabling
Gigabyte GTX 750Ti card listed above (not ASUS as the display ports won't work properly - don't get ATI cards as those have problems with the Akitio unit).
Use the automate_GPU script on this site to get the Nvidia drivers installed/loaded, and then use the DisableMonitors tool to make sure the GPU outputs properly to the external display. Sounds complicated but it's not.
Hope this helps someone.