That AMD is bothering with 12nm Polaris suggests that there's quite a while to go yet before there is enough 7nm capacity to produce high volume GPUs (e.g. Navi). It is perhaps less business risk to bear the cost of a launch of RX 590 than it is to stand still with the current product line up until Navi is ready, especially as Nvidia's Pascal line-up is still selling really well.
It's not realistic to overclock an RX480 much above 1400MHz for normal use, and even then that's at a substantial bump in power draw and still well short of the RX590 stock boost clock.
this is what I said some time ago, and is clear from the messages AMD have been communicating. Vega 20 volume is not until next year. Navi is summer next year earliest, more likely late Q3/Q4.
For starters, nvidia released Turing on 12nm for a reason. There won't be volume 7nm for large does for at least 1 year. Nvidia are hugely influential partners of TSMC, the 12nm (optimized 16nm) is largely entirely due to Nvidia's requirements for Volta and oakridge