Got a similar problem here. As i understand (i.e.) Premiere Pro uses all cuda resources for rendering and exporting. But in playback it only utilizes 1 gpu. In my case it uses the discrete 750m. I experience a huge performance jump in rendering in mediaencoder for example but not during playback in Premiere. So the question stays actual, how can i force Premiere pro CC 2015 to use eGPU for playback?
The old trick by hacking this text file is not working because there is no text file anymore. GPU is localized by premieres gpu sniffer app automatically.
Macbook Pro 11,3 (late 2013) 15" Retina
Intel Core i7 2,3 GHz
Intel Iris Pro + nVidia GeForce GT 750m
eGPU/akitio thunderbolt box tb2/nVidia GTX 970
runs stable and reliable