The input to the MC is leg pressure (in all cases) which can be modified by engine vacuum (or hydraulic assist). Stronger legs put more input into the MC, and higher pressure will be seen at the wheels (because the MC will push the fluid). I think we would all agree with this 100% for a manual-brake system.
So if a stronger leg can create higher pressure in teh tubes, why can't a 'stronger' booster?
The question isn't really whether the booster can create more pressure in the brake tubes vs without, because there were factory systems with manual disc brakes, so we know a leg is strong enough to push discs. The question arises because a power pedal has lower leverage because it was designed for a booster to compensate for that - merely to lower the required leg pressure.
Let's skip the tandem booster for now, because I cannot quantify how it works back to your specific comment of 'quicker'.
Due to simple physics, with same input pressure, a larger-dia piston will always make more output force than a smaller one. The booster is a similar thing, except it uses vacuum, and with special valving to allow it to exhaust when the pedal is released. So with the same vacuum input, a larger-diameter diaphragm will create more output force (a stronger leg). At same time, the smaller-dia drum booster will have less output force. At this point you might say 'I'll just push my leg harder to get the same force' but it may be that you would need to be 'The Rock' to get that same maximum output force with the smaller booster that would deliver the full capacity of the disc brakes.
My position on upgrading the booster comes from this:
- Drum brakes generally perform with 'duo-servo action', which means each shoe helps to push the other outward, and the rotation of the drum also contributes to that, making them more efficient (mechanically, that is -- they have other design parameters that make them lower-performing vs discs)
- I read many years ago (pre-internet) that disc brakes required higher brake-line pressure than drums, which is either due to the prev statement, or because disc pads seem to have less surface area than drum shoes do (which goes back to the piston analogy). Either way, higher pressure seems a logical requirement for discs.
- All factory disc systems had either dual-diaphragm (65-70) or a larger-dia single (71-newer) for C-bodies.
- Cost-cutting and lawsuits were alive and well back in the 60s, so there is some cost-justified reason the factory engineers spec'd different boosters between disc and drum. A solid reason.
When all else fails, I trust the engineering decisions made back in the day (and the physics of piston dia vs pressure) and run with #4.