The R9 Nanos uses HBM - high bandwidth memory - and have the best improvement vs all my other types of cards.
v6 has tremendously taken advantage of this features in R9 series that the card posting nearly 200 H/s even better past 180s H/s of 390s.
RX480s however at best posting 130-140s H/s -- could you do your magic on the RX cards?
These are 5 x RX480's .. They way past the 140 mark.. (zero gpu errors)
ZEC - Total Speed: 824.505 H/s, Total Shares: 436, Rejected: 2, Time: 00:06
ZEC: GPU0 164.904 H/s, GPU1 165.470 H/s, GPU2 164.607 H/s, GPU3 162.496 H/s, GPU4 167.028 H/s
please share your Whattman configuration.