Why nVidia is not in Macs? First of all price. Way higher than any AMD proposition, in the same segment. Secondly, AMD offers higher performance in OpenCL than Nvidia. I will not link to benchmarks again, no matter how Nvidia PR machine goes on this forum. Maxwell GPUs may be faster in games, but are not in OpenCL work.
Third is Metal. Metal is possibly derived from Mantle, from AMD. And as we know everyone got it apart from Nvidia. Only they know why. Fourth: AMD Freesync. Fifth. AMD is willing to work with Apple, Nvidia is not. OS X Drivers for El Cap have been improved for GCN cards. AMD finally cracked them. The reasons can go on, and on, and on...
P.S.
http://www.tomshardware.com/reviews/amd-radeon-r9-fury-x-power-pump-efficiency,4215.html
Fury X at -50% TDP. Power consumption of the GPU decreases from 267W to 170. FULL FIJI CHIP!
Average FPS drops for 3 FPS in 4K resolution. That means that the GPU itself was clocked at over 1000 MHz at 170W.
Fury Nano is possible.
8 TFLOPs from 170W of power consumption. If that is not power efficiency I don't know what is.
P.S.2. That means it is possible to lock full Fiji chip to 125W at 900 MHz, just as I have said few pages back.