The only thing I've found that actually gives some noticeable increase in hashrate is to adjust the memory timings in the vbios.
I have the same problem as you with some RX580s. Unfortunately I can only use the 1750 timing there so they cap out at 27Mh/s with 2150 mem clock. But it makes no difference even at 2000 mem clock the hashrate is the same.
Grab polaris and modify the original vbios yourself, no reason to download random roms from the net.
Thanks for the insight, I guess I'll have to sit down one day and really go through the process of how to find what works manually myself. I was expecting for things to work with minor tweaks as far as using a custom bios from the same model card but obviously today I see that's not the case. With both of my RX 480s dual mining, i'm back to getting 49Mh/s on ETH and 1479Mh/s on SIA. Goal is to be able to get to at least 55Mh on ETH while dual mining.