You should be able to get 28-32 mhs even on a rx 480 sapphire nitro.
Download your bios with atiflash, open it in polaris bios editor.
Copy the memory timing for the 1500 or 1750mhz to the 2000 mhz and higher blocks.
I believe you can copy 1500mhz memory timing code to 1750 and 2000 blocks.
Make sure you backup original bios first.
I don't think you can reach 28 mh's on this card surely. It's 4GB the one I got, if it were 8GB then yes possible. Right?
I don't have much time to explain so please forgive me.
I have a few rx 480 4gb cards and they do 29 - 31 mh/s easily.
rx 470 4gb can do 28-30 mh/s as well.
You need to copy the memory timing in polaris bios editor to the 1750mhz and 2000 mhz memory timing slots. The biggest gain is by modifying the memory timing / bootstrap. Second biggest gain and efficiency is by using MSI afterburner and tweaking GPU voltage, mhz, etc.
Try to change the default memory clock from 1750 mhz to target 2000 - 2050 mhz memory on 4gb cards. If it fails scale it down to 1950 mhz, 1900mhz, 1850mhz, 1800mhz until you find a stable memory clock to mine with.
The only cards I've had problems bringing memory clock up is msi rx 480 4gbs.