Hi there, I specifically stayed out of the Graphics "rat-race" until the appearance of A) Sierra and B) The AMD RX 460 (Gigabyte 4gb version), which has virtually OOB support in Sierra.
I have had good results with this combination and FCP X (the whole reason for my HACK) and the cards are less money than buying an R9 280 which i still find hard to beleive. I too have 4K monitor but wanted to edit in 4K too.
I have 23 seconds in Bruce X test
I am enthused by the apparent support of "Vega 10" chipset code that has appeared in the latest Sierra builds, which is appearing in the upcoming RX 490. This would allow me to skip the 470's and 480's (with limited support and success) and perhaps be able to use a real top-end card for FCP X at long last?
Also if this does all materialise, with having a fairly new card like the RX 460, I would have no problem selling the card without losing much cash but may not get the same result buying a 4 year old R9 280. This is just my thoughts on an FCP X build.
For anyone reading this, I am still a real newcomer to the Hackingtosh world, so don't criticise me too harshly if what i am saying is wrong, but hopefully this is a plan that i may be able to follow after reading many, many a page here on tonymacx86!