So guys Gatsby told me that the eGPU works for the rMBP mid 2015 with M370X for Windows 10 without any Software guides, too. U just have to plug in the Akitio before u start into Windows then disabling the M370X and then u can install the drivers for the for example GTX 970 (GJ Windows 10). After a restart the eGPU works fine and with less crashes than on Windows 8.1!!!
So the second case was the eGPU working on OS X (just for 10.10.3 or above) for the mentioned Macbook. Here u just install the newest NVIDIA Web Drivers or u use the tool from the Bison-Box which u can download here. http://bizon-pc.com/a/bizonbox_mac.zip then start the start.command file.
So I guess there are not any issues with the mid 2015 rMBP anymore enjoy.