It's hard to do math with too many unknown variables present. We don't know what Vega architecture changes will bring and thus any comparisons with previous gen cards cannot be made ...
What we do know is that Vega 10 is over double the stats of the 480 ACROSS THE BOARD (Which would put it on par with the 1080 TI / Titan X with just that). Then add:
-2x the Geometry IPC
-Half as much VRAM needed as before (Which could mean more effective bandwidth)
-Better memory compression
-Many more things like vastly lower latency, re-organized ROP's, etc.
Only a drooling idiot would think Vega wouldn't be at least twice as strong as the 480.
But let's be clear: I don't have a crystal ball, and I am not saying Vega will 100% beat Pascal. But if Vega 10 can't at least MATCH the 1080 Ti, it will be a Bulldozer-level failure in my opinion.