I have RX580s 8gb and the only way I got there hashrates up was via modding the bios and then adjusting the clock rates. Another point worth mentioning is that the hashrate will differ by card, memory and by manufacuturer. I have Asus, asrock, MSI and Ice amd 580s and they all perform differently in terms of power consumption and hashrate. (msi does core consumption of 79 whereas the asus duals are pushing 95-100). Good luck and don’t expect to get the same performance I say.
I had a similar problem to you though, I think maybe you havent modded it properly. Make sure you are using the right bios or try different bioses (preserve your original) to see if that solves the problem. I think it could be a matter of trial and error. Try to see how your card behaves with different settings in windows and you might see your card jumping all over the place in the undervolt settings. If this is the case, you will know there is either something wrong wtih the bios or it is just a limitation of the card. I could be wrong though if anybody knows better they could enlighten me this is just based on my experience.