Totally depends on pricing. If it's 20% faster than 980 (coming within 10% of the $1000 Titan X), and costs $550, that's a major win. If it's >20% faster than 980 and still costs $550, that's gravy.
By a similar estimation, if the 390x is 20% faster than 980, and it's priced at or above $650, that's going to be a hard sell. In this case, AMD would not be changing any paradigms, just filling a niche in an established price/performance hierarchy -- something that would be a shame and a marginal stagnation failure, in my view.
I don't think 390X will be cheap if they are investing into HBM and water cooling. But there is all this talk about HBM being super fast and the 390X having so many SPs etc etc it will be a shame if it does not out do the Titan X. If they are making the king of the hill and it comes out and it isn't the King of the Hill, I think AMD will just shoot them selves in the foot, especially since they are taking there sweet arse time to bring it out.
So it better be the fastest at any cost.