Device ID is already in one of the kexts (AMD9000Controller), but since X4000 doesn't know a "Fiji"-Device yet there's little hope it would work.
Good sign for the future though that the device id is already mentioned.
If I recall correctly the device ID first appeared with 10.10.3 or 10.10.4, but I could be wrong.
I can't imagine how Apple wants to fit one of those 400W TDP monsters in any of their tiny little cases though.