For 1080p in those games they tested it was usually 10%-50% faster than the Fury X.
(project cars seem really badly optimised for amd)
Which sounds like alot (not in the cases where its closer to 10%) but the reality was at 1080p, usually the Fury X was above 60 fps.
A 1080 GTX is overkill unless you plan to game at higher resoultions.
You probably wouldnt "feel" a difference between the fury X and the 1080 GTX, while playing at only 1080p.