It is considerable because you're using an average while it is comprised of results like this:
Or if you prefer DX12:
Fury/X isn't doing too hot lately, in many games of the last several months it is either on par or only slightly ahead of RX480 - whether this is because of it lacking VRAM or AMD not paying attention to GCN3 optimizations anymore I don't know - so I'm not sure what "sign of things to come" here means really.
And 10% was always considerable to me when anyone was ahead of anyone. 980Ti was ~10% ahead of Fury X which seems to be considerable to you since you've chosen one above the other.