I did not use dual mining to get my first results, so all figures are ETH only... 
My reply was for the person that started the thread, but it may help others too.
Since I have a 7x rx470 rig (asus strix OC 4G), I can share some settings with you.
Did you made bios mod on the cards? (copy 1500 timings to all the boxes under 1500 in polaris bios?)
If yes, then the setting's I'm using on MSI afterburner are as follows:
Core Voltage: -96 (can't go -100)
Power Limit: -50 (ETH only mining) and +0 (for dual mining)
Temp Limit: 90
Core Clock: 1110
Memory Clock: 1950
Fan speed: Auto
I'm getting 27,8 Mhs for ETH only, and 20,1 Mhs ETH + 605 Mhs SIA (dual mining)
Hope you can replicate the setup and get your hashrate up
I also have a rig woth 6x asus strix oc rx 470 (all hynlix)
I did 1500 strap up to 2000
-60 core volt
1140 core
1920 mem
Fan auto or 65%
In open air frame on powered usb 3 risers with biostar tb85
Getting 27.5mhs ethereum and 270mhs sia coin per card @ 90w each stable with a 2 month uptime
Soon as i try snd push 1950 ish very unstable