I'll add my $1.95.
Anyone who thinks drum brakes are better than discs needs to drive my '65 Ford pickup (4 wheel drums). This thing is a deathtrap, and is getting a disc swap as soon as I get the parts.
My '86 Audi Coupe, which I run in hill climbs and track events, came stock with rear drums. I swapped the rear axle for one with disc brakes, and the difference in braking is tremendous. Per the factory parts book, the proportioning valve and master cylinder are exactly the same between the two braking types for this car, and the system works perfectly.
The above post is on the right track. The wheel cylinder bore sizes are designed to apply the proper amount of pressure vs. the front (discounting the proportioning valve, of course). Master cylinder bore can also vary if a different pedal effort is desired.
I have researched the Subaru parts microfiche pretty heavily (though it was a year ago) as I want to *****can the rear drums on the Impreza, and the master cylinder is the same regardless of braking system on all of them. I don't recall finding any other parts differing downstream of the m/c either.
While I understand you wanting to question the theory of the brake types, I go by seat-of-pants examination. From my experience, the rusty, ugly, too-damn-many-stupid-little-parts drum brakes on the back of the Impreza have numbered days. They are heavy, complicated and outdated, plus they don't work as well and are hideous viewed through the wheels.
Final point of interest: I was at a school put on by a brake parts mfr and they showed us using a welding rod inside a drum just how much drums flex (you can flex them with your hands!).