I have an XFX R9 280X Black Edition and I do need to underclock it. I've tried latest Crimson and Catalyst drivers, but the built in overclock tool doesn't affect the actual clocks under load. The manual fan speed control does work though. After a bit of searching, I tried MSI Afterburner and that also doesn't budge the clocks. Any ideas? And yes, I did hit apply after setting the clocks.