Polaris 10 from what little info they release, should perform higher than 390X. 390X can't sustain 60 fps in Hitman at 1440p, even in DX12.
So my guess are Polaris 10 #1 $349 giving ~Fury performance, while Polaris 10 #2 $299, giving ~$390X performance.
So they would raise the bar, $299, 290/970 -> 390X performance. Not by much though.
I don't expect them to be priced lower, it would be too generous and they do need $$ badly.
Polaris 11, no way will it match 290/390 levels. ~120mm2 die is just too small for that kind of performance levels.
ps. Ofc I could be very wrong, and AMD suddenly becomes a charity and price 390X beating performance for $249 or something and 390X class performance for $199.. if they do, I would immediate get 2x Polaris 10 for a CF setup.
Edit: For rough performance estimate, you can calculate it on die size: 232mm2 * 2.4x density = equivalent of ~556mm2 GCN on 28nm. Add uarch improvements, there is potential there for ~Fiji performance. So it should land above 390X easily. For Polaris 11, 120mm2 * 2.4x density = equivalent of ~288mm2 GCN on 28nm. Well below Hawaii, which is ~438mm2. With uarch changes, it can approach Tonga, 380X performance. No more than that.