I was wondering why I got tearing with XBMC even though I had enabled vsync and why XBMC was reporting a refresh rate of 50Hz for my screen instead of 75Hz as it should have been. It turns out that NVIDIA’s DynamicTwinView feature reports a fake refresh rate (that of the MetaMode which encompasses all of the screens involved in the TwinView) which may not be the one for the physical device. As I am not using TwinView I added the following Option to my xorg.conf Device section:

Option "DynamicTwinView" "False"