If we have 28500mm2 in total piston area, 55%from it makes 15650mm2 up front and 45% 12850mm2 at the rear.If we presume 2 x 4 piston calipers we have 8 pistons at 1956mm2 each front and 1606mm2 rears. This makes 1,78 pistons rear and 1,96 front. Lets make it 1,3/4 and 2 inch. But we may use 5/8 master and size down in caliper pistons accordingly.
We should not use more iron in the disc than nessesary as the unsprung weight is getting high then. It is better to fabricate a proper disc cooling system. Pad size is very much given by the selected piston area. Therfore a suitable pad material should be selected for proper pad temperature.
Exept for temperature there is no difference in braking power with different pad area.
If a stiffer rollbar is mounted on the front axle, this axle is transfering a larger amount of the total weight transfer. Making a big difference between the left and right tire. This makes the car to understeer, or getting less grip. The same goes for the whole car when it comes to braking. The weight is just moving along the car in place of sideways.But here BOTH sides transfer the same amount of weght over to the front axle. Making a big difference between the front and rear axle, making the whole car to loose grip. We can NEVER brake harder than the grip of the tires. Therfor we should pay great attentione to brake bias, car balancne and proper tire size and wheel alingment.
The last lines was maybe a little of topic, hope you dont mind.
Regards
Goran Malmberg