I'm having EXACTLY the same issues. Some of my cards will do 28Mh/s, some 25. No matter what I set, the miner ignores the values.
I have re-flashed, no change.
Highly frustrating.
So I've found out that the OS is causing it, I installed simplemining and on linux there is no such issue.
Practically, if you OC in Win10 and it crashes , some "auto-protect thingy or bug" gets turned on and you can no longer OC the memory properly
Now I need some good bios for rx480 4gb:) since most custom bioses are made for the 8gig version.