Fall 2017 (Scorpio) launch makes me think it might use Vega's updated GCN. If Radeon RX 480 is closer to 5 TFLOPS (AMD said 'more than 5 TFLOPs') then I think going with a larger Vega based custom GPU at sane clocks makes more sense to hit their 6 TFLOPS target.
I thought the 6 TFLOPs number was for the entire system, so it includes the CPU as well, which doesn't account for much but probably adds about .15 TFLOPs.
Polaris has just been listed at >5 TFLOPs, so they wouldn't need that much extra hardware on top of what you'd get with a 480 and you can always increase the clock speed as well, which by the time this thing needs to come out shouldn't be a problem as the process matures.