I just used this guide https://steemit.com/ethereum/@yogibear/mine-more-ethereum-with-rx470-rx480-bios-mods

Step 1 get the bios from the card using atiflash
Step 2. use PolarisBiosEditor to edit the straps
Step 3. Flash the bios to the card.

I modded the core voltages, straps, fan speeds and power limits. if you don't want to do it yourself just down anorak https://anorak.tech/t/anoraks-vbios-collection-optimized-settings-for-performance-power-saving/13

You can boost the mem speeds to get some more hash. but it makes the cards hot so it is up to you.