I have the same Combo.
Welcome to the Club.
Do you also have the same problem? If yes - can you please describe some details, e.g. which version(s) did work correctly with Freesync-ultimate and which versions do not work?
I Have the same Issue.
17.12.1 is the last Version for me where FS over DP is working without Flickering.
I tried so much. I fidled around with the FS Range via CRU, i installed every single Driver Version, turned on/off every point in the Drivermenus.
I used HDMI and DP, bought a high quality DP Cable...Hell i even disconnected my old Soundcard from the System.
Nothing relliably helped. The only thing i can do is to return to 17.12.1.
I hate this Issue and honestly im considering to sell my Display and Vega and return to NVidia.