Something about AMD's next gen cards is not adding up. I'll explain.
If we go with the idea that Polaris 10 is a 2304-2560 shader card and it improves GCN 1.0/1.1 perf/watt 2.5X, and that it ONLY matches a 390, then things start to not make any sense for Vega.
We have a rumor that Vega is a 4096 shader part if Polaris 10 is a 2304 one. Recall that Pitcairn and Tahiti had similar GPU clocks for the most part.
HD7870 = 1Ghz
HD7970 = 0.925Ghz
HD7970 = 1.05Ghz
That means
if Polaris 10 ~ 390 = 62%
Now assuming perfect scaling for Vega and similar clocks to Polaris 10, 4096 / 2304 = 77.8% increase in performance. Following along, that means Vega = 390/Polaris 10 62% x 1.778 = 110%***
That means Vega would only be about 78-80% faster than an R9 290X/390.
Ok still following me? Didn't we earlier state that Polaris 10 has 2.5X perf/watt over R9 290X/390 but now with a 4096 Vega we only ended up with a card that's 78-80% faster than a 290X.
However, AMD has Vega with even higher perf/watt over GCN 1.0/1.1 than Polaris 10 has........we have a logical inconsistency.
It also means that Vega 10 would only be 110 / 80% = 38% faster than Fury X. That's not fast enough.
Working backwards instead:
290X = 61% x At least 2.5X increase in performance for Vega 10 = 152%
152% / 1.778 = Polaris 10 = 85%
That's why it's not adding up. If OTOH, we start making extra assumptions for Vega like it having 128 ROPs or much higher clock speeds than Polaris 10, the performance delta between a 2304 Polaris 10 and 4096 Vega would grow even more. This would become too unrealistic if Polaris 10 is a mid-range card. It would then straight up make Polaris 10 a low-end card for the next gen AMD stack.
The only way any of this makes sense if is Polaris 10 is much faster than a 390, Vega is clocked way way higher than Polaris 10, Polaris 10 isn't mid-range but more low end priced even lower than $249, AMD lied about 2.5X perf/watt estimates, or Vega is a much larger chip than 4096 shaders / 96 ROPs.
***Something else. If NV improves perf/watt just 1.6X over 980, 980 = 67% x 1.6 = 107% (29% faster than the 980Ti), then our hypothetical Vega 10 is only as fast as a flagship GP104 = almost an inconceivable scenario for a 1TB HBM2 flagship Vega. This is another reason, the 2304 Polaris 10 = 390 and 4096 Vega 10 estimate make no sense.