Its my understanding that the RX580 and other higher end RX500 are actually a "new chip" (not an overclocked old chip). However, the new chip is an incremental step on the old one. It fixes some issues and runs a bit faster. RX500 series is NOT a significant upgrade for folks with an RX400 series card.
Vega is due right at the end of May -- some say beginning of early June.