What exactly are the symptoms?
Are you sure the GPU acceleration is working? You can bring up an overlay menu that will show GPU and CPU utilisation.
Secondly, did you play with the movie/monitor sync options in XBMC? It fixed the final small glitches in playback for me?
There is a significant amount of tearing. I can see black lines where this is happening. The video also appears to be stuttering slightly but this could just be a symptom of the the tearing.
How do I see the GPU usage? I do suspect this is the problem. Each core of the CPU is working at around 50%. I checked this from the debugging overlay.
I have played around with the sync settings but no fix yet. But will keep trying this.
I am also going to clear the drivers and try a manual install.
EDIT: Appears to be fixed. I enabled the Tear Free feature in the Catalyst Control Center (believe it or not). Now I just need to get a 1080p to truly test it, have been testing on 720p.