Potentially if big vega is time as fast as a 480 and with the lead amd have with a sync and dx12, it could easily beat the 1080 and nvidia won't be happy about not having the fastest card for a year.
But they are releasing the TX, so that will definately be the fastest card, I do not think Vega will be much faster than a 1080. Probably the big one that is some time in 2017 will be faster but that is a long time.