If you're looking to do primarily CUDA/OpenCV work you'd be crazy to not build your own PC/drop in a new Pascal card. $450 will get you a GTX 1070 that should easily do 7 TFLOPs of SP (w a slight overclock), and for $600-700 a GTX 1080 that could push 9+ TFLOPs.

As far as price/performance, a high end Mac is only worthwhile if you have very specific software requirements (mostly, if you use Mac-only media production software).