Yess, this hash rate is pretty normal.
You can try to increase the memory frequency to improve your results and also underovlt a little bit to reduce power usage.
Since you are pretty familiar with bios flash right now, you can also try to modify the bios yourself with polarisbioseditor and lower the timings. It's pretty simple and you will find guides or even videos on youtube.
With everything properly configured you should be able to archive at least 29Mhash with that card
I'd be glad to hear what stripes were used, i did exprement as well with this card and really interesting case - few cards work at 28mh others downgraded to 18mh.... with same bios under ethos without any additional changes (default clocks 1180 and 2000). Was wondering why cards do act differently, maybe smb tryed and got it more stable?
maybe smb knows how to undervolt rx480 cards under ethos?