Definitely yes.
RX 480/580 is like 6 times faster than even 1080ti in some FCP task.
I believe 9to5mac did some test.
I personally have rx 480 in one of mine hackintosh and it's working perfectly on latest Sierra.
Hey, great conversation you're having here. I am a longtime user of FCPX, and I currently (as you can see) rock a GTX 970 in my rig. I plan on (at a minimum) upgrading to Z370 board (preferably Gigabyte), buying an 8700k and an RX580.
Is setup/macOS installation with said components difficult? Is there anything I should know before going about doing this? Will my RX580 work right out of the box (I know there is native support for the card in High Sierra... but to what extent?) Also, is it possible to enable Intel quick-sync/boot with the iGPU (if you know anything about that?), and if so, how?
Any response is much appreciated.