Noticed the same behaviour with my 4850 - the core clock speed remains at the idle underclock speed of 160MHz throughout the entire game session, confirmed using GPU-Z sensor logging. This doesn't happen with any other games I have tested, just CoD.
I'm running v10.10 of the Catalyst drivers and don't see the force clock speeds option either. However, I was able to force the clock speeds using the ATI Overdrive option in ATI Tray Tools (
http://downloads.guru3d.com/download.php?det=733).
Unfortunately, forcing the clock speeds didn't seem to noticeably improve performance.