260X works in Yosemite Public Beta by adding it's device id to AMD8000Controller.kex and AMDRadeonX4000.kext.
there is no support for R7-260x in yosemite final release260X works Apple's next OS..
Mr Fl0rian , unfortunately Clover not worked in my System . with 10.10.5 I get the card to work, but GPU clock speed is very lowThe R7 260X should work fine in Yosemite with Fake ID 0x665C.