None of them will. Vega is the one you want if you are coming from Fiji. Polaris is more 290/290x/390/390x, while Vega is going to be the high-performance part.
I don't think that will be the case. If it is, that is a huge fail by AMD.
New architecture and a doubling of available transistors even on a much smaller die and still can't beat fury x? It surely will by 20-30% and that will be their 490x. The 490 will likely be 10-20% faster. This should be worst case scenario.
Vega should be close to double the speed of Fiji. Surely 70-80% better at the least. Again, newer architecture, bigger die size to polaris and hbm 2 on top of that...