This link shows a comparison between a P25 back and a P45, both shot with a Hasselblad 80mm at f16.
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Diffraction probably means that f/16 is too small an aperture to use with the 6.8 micron pixel spacing of the P45, at least if you want maximum sharpness; f/16 is about at the diffraction limit of the 9 micron pixel spacing of the P25 sensor, so diffraction might prevent any lens or sensor giving resolution significantly better than the P25 already does at that aperture. The rule of thumb seems to be avoiding aperture ratios more than twice the pixel spacing, like Thom Hogan's observation of needing to stay at f/11 or below with the D2X and its 5.5 micron pixel spacing.
The Olympus E-1 has a Kodak FFT CCD related to that of the P45, with the same 6.8 micron pixel spacing, and the E-1 will not automatically choose an aperture beyond f/11 in S or P mode, I believe due to diffraction effects.