The RX 5xx series is not a rebrand.
They are being made on an upgraded process that is reported to be a little more efficient (no hard figures yet though) and will allow them to be clocked about 5% faster than the corresponding RX 4xx series part.
 I'd guess probably 5-19% lower power at the 5% reported clock increase, but I could be WAY off on that power guess.

 (EDIT - turns out the new process clocks higher but is a hair LESS efficient, power usage went UP on the 5xx series more than the clocks did).

 As far as hashrate goes - it will depend on the coin, memory-limited stuff like ETH probably will see no or barely any increase, core-limited stuff like ZEC should see a small increase.

 Also, AMD has indicated that VEGA will not be targeting the mainstream where the existing RX series lives anytime soon - they're targeting the high-end, where the Fury/FuryX/Nano used to aim at (edit) and the GTX 1070 and up live.


 I suspect that there will be a short-term "new card availability limits" price premium for the RX 5xx series, but after a couple months they should settle in at the same pricing we see now on the RX 4xx series.