It is even hypothetically possible to use a video card in Windows via Boot Camp even if there are no OS X drivers for it! You would need to install the drivers in to Windows first which means initially using a different video card supported by OS X so you can install Windows via Boot Camp. 😟 This is purely down to Apple's pitiful drivers. For example I have an AMD Radeon HD 7950 card, this card supports audio via the HDMI cable in Windows, it also supports 30bit color in Windows but does not support either of these features in OS X even on the same Mac Pro. It is a sad and ironic situation that many video cards work better in Windows via Boot Camp than in OS X even on the same Mac Pro. The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 12 clicks from bottom or 75. Again unless you have custom Mac firmware you will not see anything until Windows has finished booting and therefore loading the Windows driver for this card. Testing conducted by Apple in April 2015 using preproduction 2.2GHz quad-core Intel Core i7based 15-inch MacBook Pro systems with 256GB of flash storage and 16GB of RAM. With regards to using it in Windows via Boot Camp, yes this will work. However unless you or someone has installed custom firmware on to the card it will not show anything until after the Mac has finished booting. It is my understanding that an R9 270 card is supported by El Capitan and hence presumably also Sierra. The only drivers for AMD video cards for OS X are those built-in to OS X and written and provided by Apple. There are no AMD drivers for AMD video cards for OS X, there are no hacked drivers for AMD video cards for OS X.
