I think it depends on what kind of clock speeds they can get out of the chips they have. AMD isn't at the top of the market with Polaris 10 so they price their cards relative to the rest of the market and past offerings from the previous generation.
If they had a card that could perform for $350 of performance, that's what it would sell for, but if it can only justify $300, that's where it would be. For $350 I'd expect something that trades blows with the 1070, because eventually it will drop close to MSRP and at $350 you can't sell a card that's going to have more than a ~5% performance gap or most people will just pay the extra $30 for the added bump.
We'll eventually see a full die, but I expect that AMD might be waiting until they can get better clock speeds out of their chips. An extra ~10% performance from a full die isn't worth $100, so it likely needs a good bump from clock speeds as well, because for a 50% increase in price (or perhaps closer to 30% given that a 480X wouldn't ship with less than 8 GB) you'd expect at least a 25% increase in performance.
The real question is who's going to get to that magical $300 price point first as right now with the 480 at $200 and the 1070 at double the cost, there's a gaping hole in the middle and as the 970 proved, that's a sweet spot to own in the market.