Grigg said something interesting about boosted brakes in another thread. It takes a certain amount of line pressure to lock up any individual brake. You may or may not be able to produce that amount of line pressure, even with a booster, if your brake hardware is worn or mis-matched.
Saying that " . . . when a brake is locked up the tire determines stopping distance . . ." is a little misleading. When the tire is locked up, even on dry pavement, it does not produce as much friction as it did just before locking up. On dry pavement, tiny particles of rubber are torn away from the tire surface and actually act as a low efficiency lubricant. I think that a lot of people have noticed that, without ABS, the place where your vehicle will stop after a skid is difficult to predict. That final resting place depends on a lot of factors, but, barring outside interference, (telephone pole, other vehicle, etc.) it will probably be noticeably farther down the road than is expected.
Dual master cylinders are a big safety factor. If you have a single chamber master cylinder and pop a brake line anywhere, you are left with the emergency brake. With a dual master cylinder, no matter where the brake line fails, you will probably have a lower pedal, but functional brakes on at least two wheels. That's a big selling point for me, especially on an old vehicle. Don't get carried away though. A quadruple master cylinder is almost sure to make emergency stops very exciting.
I may be fooling myself, but I would prefer to stick with non-boosted brakes unless it is impossible for me to lock them up. I have to admit that I have a tendency to get on the brakes, hard, when things go wrong. That isn't always the best idea. A few weeks ago I had a trailer jump the ball. Locking up the brakes would probably have been a bad idea.
Finally, if you have a dual master cylinder setup, I would always recommend a proportioning valve. I just haven't seen many vehicles that have exactly the same brake setup front and back. Even if the vehicle has the exact same brakes front and back, they probably won't have the same braking effect because of weight transfer. Identical brakes front and back with a 60/40 weight distribution means that the rears will probably lock up first, if I remember my physics.
