Well perhaps TV size/technology is a factor too. This was on a 65" Plasma. The frame drop was during an action scene in Iron Man and trust me it was very very noticeable. At other times the frame drops are less noticeable depending on what is happening on screen.
I did find a good workaround to the problem which was setting XBMC to sync to source AND display (with audio sampling)... this resulted in zero dropped frames... unfortunately when XBMC got HD audio introduced via patch this no longer worked.
So, I bought a 6450 and now experience zero dropped frames with just "sync to source" being set.