not all R9 280x are unlockable. My 280x are the MSI version and i believe (from what i remember researching when i bought them) i could only find that the saphhire and MSI ones were unlockable.
I use MSI afterburner to undervolt my MSI 280xs and AMD catalyst to undervolt my 290s
Could you possibly share your MSI 's BIOS? My MSI 280x cards are locked.