Here's why it depends on the game:
http://www.guru3d.com/articles-pages/gears-of-war-4-pc-vga-graphics-performance-benchmark-review,1.html
quote"
"The last graphics card is the Mc Daddy that is bound to show CPU bottlenecks due to it's massive horsepower, the Nvidia GeForce Titan X (Pascal) is way ahead of its time and priced 1300 USD alone. Up-to 2560x1440 you are fine. But sure at 1080P we finally run into CPU overhead. Honestly that is expected. Overall we can conclude though that up-to say a Radeon Fury X or GeForce GTX 1080 an AMD FX 8370 now is sufficient enough to run the game. "
I currently run a FX8350 @4.5Ghz,I would say I'm pretty much at the limit with Crossfired 7970's(R9280X),I ran the same GPU's with a Phenom II 1100T@4.0Ghz with the same results.
so In your case I would stick with a single RX480.
I have also been running crossfire in every series since the 3870's and have had 0 issues.