You are not alone with this issue
I recently bought a Crosshair VI Hero motherboard and my Fury card can't output 4K@60Hz with it.
Same combination of graphic card + DP cable + monitor works fine on another machine
Issue persists with latest driver (17.3.3) + Windows 10 up to date
Mine also shows 2.7Gbps x4.
Nearly a month later and I have this issue as well
1700 build using a Crosshair VI Hero board and a R9 fury.
Dump the fury in my old i5 system it does 4k 60hz with DP cable np.