AMD did their 28nm product stack pretty quick.
6 months is a long gap between Polaris and Vega, and the reason I think, is HBM2, the stacking process itself adds months to the production line.
If GP104 is on GDDR5, performance isn't going to be as great as some folks hope. Even the 970/980 now is already bandwidth bottlenecked, it needs VRAM OC to scale. What's their angle?
1. 384 bit bus on GP104? Would be unprecedented but definitely possible. Basically 980Ti class bandwidth. With improvements in prefetch, cache and compression, they can hit a slightly higher performance profile than the 980Ti with GDDR5.
2. 256 bit bus, performance would be very bandwidth bottlenecked, in games where the compression is efficient, there will be good performance, otherwise it's < 980Ti.
Both of these are not ideal. The ideal is 256 bit bus + GDDR5X. But this means delays to release, paper launches etc. However, when NV is so far ahead, and their current GPUs sell, they can afford delays go for the ideal scenario. GP104, 256 bit bus + GDDR5X will be a great performer while being cheap to manufacture. Win-win scenario.