For all of those who are mining with locked R9 280x's and have windows + msi afterburner, try this!
[Settings]
VDDC_IR3567B_Detection = 32h
VDDC_IR3567B_Output = 0
MVDDC_IR3567B_Detection = 32h
MVDDC_IR3567B_Output = 1

That should be in your MSI AFT profile, should unlock your card, at least worked for my MSI Gaming 280x's.