ASSUMING you can't without exhausting the possibility is lame; you are no more certain than the questioner, you're just biased differently than he is. Personally, I'm interested in knowing if I could put the 750m Furthermore, if/when you replace a MBP GPU that failed due to the $@$*!& new compound used in lieu of lead solder immediately improves the longevity of it. Its not like the GPUs of everything BEFORE 2007 failed. I'd just look for the model with the highest performance, supported kext and compatible pinout/stencil... if that doesn't work, I'd work backwards.
You can buy an adapter from M.2 to MBPr or Air for about $20... http://eshop.sintech.cn/storage-adapter-c-130.html The REAL question is if the NVMe protocol will be a hiccup (as in, I don't personally know that answer) ... and if it'll get the full speed of the 950. In principle, I don't see why it wouldn't. And I WILL be testing this out.
