Despite what Apple says the R9 395x is a lower baseline card by todays standards. While 4GB VRAM is better for higher resolution displays the amount of VRAM is a single factor in it's performance. The memory bandwidth of this card vs. mid or higher level cards will play a significant role in how quickly the card can utilize that RAM and it's on the lower/mid spectrum of it's generation of cards, and that was a previous generation, newer cards from Nvida and ATI have made significant leaps and continue to do so to meet the demands of technology, and this includes the increase in display sizes and the need to utilize this is sufficient hardware. While it's never a good time to buy technology because something bette is right around the corner the video card/GPU industry made massive leaps this spring and summer and previous hardware was substancally depreciated as a result.
Metal is a developer language for graphic applications, it is not a driver for the display. Unless OS X utilizes metal for the applications you are using it's performance claims are not applicable. Keep in mind Apple prefers to develop their drivers in house rather than let the developers of the product develop drivers for them. As a result there are fewer developments for this card (and other ATI or NVIDIA devices) as their would be for Windows and Unix users. Apple may choose stability over performance. And P.S. if you updated your ATI drivers from ATI for bootcamp and the game is tearing up in windows and frog-fart speed on OS X that is not out of the question as to ATI making their own drivers and Apple forcing you to use what they offer for oS X
