My Results:
Mac OS v10.12.6
FCP v10.4
CPU: Intel Core i5 7600K 3,8GHz (OC with Gigabyte "Auto" features to 4600MHz) - Cooler Hyper 212EVO
MB: Gigabyte GA-Z270 Gaming 3
RAM: DDR4 Corsair Vengeance LED 2666 2x8GB
GPU: Gigabyte Aorus RX580 8GB
Bruce X 5K Render Test Time: Less than 15 Seconds
Fun fact, I'm only getting half of the video rendered. The horizontal left half is black...
I tried MacX Video Converter and says "NO" to hardware encoding support, then I search and found this thread:
https://hackintosher.com/guides/fix-rendering-issues-fcpx-high-sierra/
Enabled iGPU and eGPU on BIOS, MacX Video says "YES" to hardware encoding support, but performance is poor, about 1min 25sec to BruceX 5K and bad render, left half of the screen Green, Right half of the screen correct.
So I revert to Auto GPU on BIOS settings and everything works fine, maybe was my fault cause I'm on Sierra no HighSierra