it will run 2x faster, so even if it cost $400 it's worth it, actually it should be better because dual chip consume less than a single one, while maintaining the same hash
Why are you assuming the RX 490 (if it appears) will be a Vega card?
The rumors of when the RX 490 is likely to appear are ALL WRONG for a Vega card (2+ months too soon).
*MY* bet is that the Vega cards won't be RX 4xx at all for naming anyway - might be RX 5xx, might be something else.
As I already SAID, the Vega is more an update to the FURY architecture anyway, not to the Polaris - the use of HBM is definitive.
The TECHNOLOGY however - 14nm - IS the same as Polaris, as the chart you posted clearly shows.