Finally got around to measuring the brake pedal and booster diaphragm plates.
According to my rough measurements:
- Brake Booster Diaphragm Plate Diameter: 7.87 in
- Brake Booster Piston Assembly Bore: 2.36 in
- Total Brake Booster Diaphragm Surface Area: 88.625 inē
- Pedal Ratio: 3.75:1
- Brake Master Cylinder Bore (front): 1.0 in
- Brake Master Cylinder Bore (rear): 0.874 in
This means that the booster adds about 870 lb of assist to whatever you're stomping on the pedal at. Assuming 100lb leg force, effective force into the booster is 375 lb. Force out of the booster onto the master cylinder piston is ~ 1245lb. This means front lines pressure will be ~1586 psi and rear line pressure will be ~2076 psi.
If I run build the pedal set for a 6:1 ratio, use a 0.75 in master cylinder for the front circuit and a 0.625 in for the rear circuit, it will take an additional 17lb of leg force to get the front line pressure to where it was stock (~1589 psi). The rear line pressure, at this leg force, will be increased by ~10% to 2288 psi.
Comparing the Cayenne calipers to the 325i, at these new line pressures resulting from a 117lb leg force, there would be an increase in clamping force in the front of roughly 8% and and increase of roughly 5.2% in the rear.
All that to say; I think the balance delta shouldn't be noticeable to the meat in the seat; and if it is, it can be dialed out with bias adjustment.
EDIT: on the stock master cylinder + booster (assuming DSC/ABS module doesn't have any built-in bias), this same setup would result in a 7.7% increase in clamping force in the front and 4.5% decrease in clamping force in the rear. This would definitely upset the balance, for sure.