Alright I have done some test from both R9 390 compare to RX 480.
There is no performance Increased which leads for all R9 300 and RX 400 series useless running Crossfire on Battlefield 1.
how ever single card both R9 and RX does seems to take effect on the FPS on DX11 and DX12, which is weird from both single and crossfire point of view.
I did not just try DX12 on Crossfire mode I also did it on DX11 and there were Performance Reducing issues.
as I know that DX12 dont have MultiGPU handling only single card. still Crossfire on DX12 turns out to be playable. "weird"
on DX11 in other hand which have for some reason a huge Performance issue.
This is a problem for those who use 144hz monitor active.
Freesync turns out great for both R9 and RX on single card.
Please submit a report here
It is well known problem.At this moment crossfire must be disabled for BF1.
I haven't saw any working solution for this issue. Maybe new patch for BF1 or next drivers will fix this problem.