A simple fix has been discovered to fix the clockspeed issue. It is because of the flash Ubisoft logo video that plays when you start the game which confuses the card.
Simply rename or delete the Ubisoft logo video found in \Steam\steamapps\common\il-2 sturmovik cliffs of dover\parts\core\GUI.
It has fixed the issue for my HD4870 and alot of other AMD users!
