The R9 XXX name is only a cosmetic issue, it gets changed by the latest clover to RX 480 but doesn't effect performance.
Something you could try, Intel fake ID 0x04128086, and ig platform 0d220003. If you do this you should be able to inject ATI to get RX 480 name corrected. If you dont inject anything Intel or ATI and use these numbers the R9 xxx name will come up but your performance may improve.
When you get to clover boot screen, before you hit enter disconnect cable to CPU graphics or Igpu. Hit enter and if you get to log in screen the RX should be fully supported and run the highest benchmark scores. It may work for fcpx as well as long as you have the igpu with metal support by adding fake ID and ig numbers.
I use these#s on my g3258 Haswell pentium rig for igpu and it works well, not sure about the 4790k, but it's a Haswell.