Actually not.... A 1600mhz FuryX is faster than the GTX1080, let alone a brand new core design.
If we take one of the new custom 1080s we can see the FuryX is about 67% of the performance at 1050MHZ. 1600MHZ is a 52.3% overclock, so 1.523X67 = 102%. The regular 1080 is at 93% for comparison. Therefore, best case scenario for a FuryX overclocked to 1600Mhz puts it at a very similar performance level, although performance would never ever scale linearly like that . To close the gap to the 1080Ti requires the architectural improvements.
This should all be possible, but does indicate that best case for Vega wont be absolutely humiliating the 1080ti, maybe 10-15% faster? that is good but does mean if you were to buy a 1080 or 1080Ti now you don;t had a lot to worry about. maybe you could wait a fe months to get a Vega 10-15% faster at similar prices, its hardly game changing.