As Raevenlord said... "Remember overclocking gains aren't linear, though, and since it seems the RX 590 will still make use of GDDR5 memory (2000 MHz clocks; and again, remember the investment in repurposing the design for GDDR6), so you better push out your own manual memory overclocking to improve on Polaris' most pressing limitation."
Without an improvement in memory through-put/bandwidth, I don't see the Polaris 30 actually giving us much more in sheer performance. While I'm sure AMD has already developed and allocated funds to have a GDDR6 memory controller it's just how easy it is to bolt to the old Polaris.
While now I'd say we won't even see any Vega architecture implementation with GDDR6 for any discrete desktop GPU either. All this has me thinking it's all about Navi now. I almost consider if AMD has a Navi GDDR6 that ready for mainstream, and this is just some "pipe-cleaner" placeholder to buy some time?