Here it is:-
http://www.dxomark.com/index.php/eng/Camera-Sensor/All-tested-sensors/Pentax/645D
While the IQ of the 645D is higher than average, when compared to the majority of mainstream DSLRs, it does no better than the 135 Full Frame Nikon D3X nor the MF Phase One P40 (which is of about the same resolution). See this Comparison.
In fact, the 645D loses in all 3 major aspects under test, namely, colour depth, dynamic range, high ISO performance and hence obtains the lowest score amongst the 3 cameras in the comparison.
Related:-
Is MF Really Needed for Higher Pixel Count?
Comparison of Sensor Specs - 645D Against Others