http://www.scirocco.org/faq/brakes/p...n/pfpage2.html
Have a read of that. The bigger the master cylinder the more force you need to create the same amount of pressure in the lines, but a smaller one may not have enough fluid in it to fully work the brakes, and the larger brembo calipers do require more fluid
Based on that, I'm guessing if you had near new rotors and pads there would be no need at all to go to the bigger master cylinder.