All good stuff. There is another thing to consider. For maximum efficiency, and stability, the fronts must lock up just very slightly before the rears, so the front to rear balance must be considered. We know that you don't "need" discs at the rear of a Mini, indeed the drums need a limiter valve or very narrow bore wheel cylinders. However, by using very small discs, and calipers with small diameter pistions at the rear, you could create a situation where the front and rear brake temperatures remained approximately equal, so the balance did not shift about as the fronts alone became hot, and the overall brake efficiency and maximum achievable deceleration would be optimised. Probably not practical right now, as lots of development work would be needed.
But if the problem is that the fronts are not able to lock, there is a fundamental, but simple, problem. You either need more hydraulic pressure (stronger right leg?), larger effective disc diamater, or larger diameter caliper pistons. Now a larger effective disc diameter may be possible with a multi-piston caliper and long, narrow pads so the contact area is as far as possible towards the outside edge of the disc. If the total piston area is the same as the originals, the master cylinder will still be able to displace sufficient fluid volume and will not need to be changed. But surely it is easier just increasing the disc diameter?
As it happens, I prefer 10" wheels, and those limit your choice severely!
Larger diameter caliper pistons are of no help, because to meet the safety requirement that there is always sufficient pedal travel to apply the remaining brakes fully if one circuit fails, the master cylinder diameter has to be increased, so you are back to the same hydraulic ratio.
As has been stated here many times by others, a servo does not improve braking efficiency, which is purely a function of friction surface geometry, thermal properties and applied pressure. However, if the problem is simply that the maximum pedal pressure that you can apply is not generating sufficient hydraulic pressure to lock the wheels, a servo will help, provided it is large enough to apply sufficient force itself. Many are not, and run out of vacuum differential pressure under very hard braking. A longer pedal, i.e. larger lever ratio, would achieve the same thing, if it was physically possible and you could accept longer pedal travel, remembering that full master cylinder piston travel must still be achieved.
Which brings us to the possibility of a quick test. Make a bolt-on pedal extension (doesn't matter if it is unsuitable for serious road use at this stage) and try using it to apply more pressure on the brake roller test.
Realistically, the only short-term way forward, using standard Mini parts, is larger discs, which with the same pad material and caliper piston diameter will give you more braking torque, as I think has been said above. The pad area has little to do with it, as the friction is ideally proportional to pressure alone, and independent of speed or area. But the pad area is very important for heat dissipation and wear, so bigger is generally better, within sensible limits.
Long term, there is still scope for improving Mini braking in various ways, including solving the front/rear balance problem by using ABS, but that needs vast development resources. You can't design such a system at home, or just throw in something ripped out of a modern car.
My brain cell will burst ... 
