Compliments of Mathphotographer, here is a short but detailed look at the differences between Q2 Monochrom files compared to Q2 B&W conversions. As we would expect, the Q2 Mono has a huge advantage at high ISO shooting in terms of image quality.
If you are seriously into black and white, the Q2 Mono is worth a hard look; 47.3 MP, ISO 100,000 capability and the 28mm Summilux lens make a very strong case for the Q2: