I dunno. AMD seems to have gone out of their way to make TressFX work just as well on Nvidia's hardware as their own. Nvidia's HairWorks on AMD hardware? Not so much.You can't blame nV for that, its their software, they aren't going to go out of their way to make it better on other vendor's hardware, that would be doubly counter productive on their end. They would have to spend the money for the optimization for other vendors cards, and it helps sell the other vendor's cards. What company in their right mind would do this, unless they are in a position that adoption of their tech is too low to benefit from?
AMD provided the code to NVIDIA when they created TressFX while NVIDIA is only providing .dlls for game/HairWorks for the Witcher. AMD devs can't see what's going on and optimize for it yet Nvidia has full source code access for TressFX.