Here's what I wrote in the support ticket:
"This is a new computer build that has been working prior with a Sapphire  HD6850. I just bought a HD7950 to upgrade, and along with that, I also  did a hard drive format and a complete reinstall of Windows 7 x64. So  this is a clean machine running with a very minimal of software on it.  
My issue is the HD7950 keeps throttling to 850mhz during gameplay, but  it should be 925mhz. The temps are in a very good range, never exceeding  60c on full load for over an hour of benchmarking.   I went into Catalyst Control Manager and increased the Power Setting to  +20, which I was told would make the card work at full speed. But the  card continued throttling to 850mhz.   
So I tried disabling the Overdrive settings in Catalyst and using Trixx  instead, and upped the Board Power setting to +20. However card  continues to throttle to 850mhz.  
So then I reinstalled everything including AMD Catalyst and Trixx, and  when trying again I get the same result - the card continues throttling  to 850mhz during gameplay.  
And finally I tried one last thing, I disabled CCC on startup, so only  Trixx loads when my computer boots. With this, I was able to maintain  the 925mhz full speed during gameplay. But now I am unable to use my  Catalyst Control Manager, because if I load it at all, my card starts to  throttle again. The only way to avoid throttling was to disable  Catalyst which is a harsh workaround.  
Can Sapphire support help me figure out why my card is acting this way?   Thank you,"