Originally posted by photoptimist The short answer is that a pixel shift color shot converted to B&W (for a perfectly stationary subject and solid tripod setup) would be virtually indistinguishable from a monochrome sensor shot on both sharpness and ISO.
The longer answer is that the slight attenuation of light for in-band color (e.g., the Bayer filter does not transmit 100% of any color), a painstaking statistical analysis might find a small fraction of stop ISO advantage for the monochrome sensor with red or blue subjects. However, due to double sampling of the green pixels, the pixel-shifted Bayer sensor might show a small fraction of stop ISO advantage with green subjects. The effect would probably only be about 1/4 of a stop at the max - can one really see a difference between images shot at ISO 1000 versus ISO 1200??
If you employ pixel shift with four images on a monochrome sensor, you cut noise in half or re-gain a full stop.
These little rgb filter really influence results. That said, I would not buy a monochrome camera only to shoot super high ISO with pixel shift.
It is all about clean images right from the sensor, the ability to use color filters to separate bands and stack composites and some extra fun, like faster base ISO, easier processing.