Here is my take on this:
More grip is a consideration as much as bigger discs, ultimately the braking force you can apply to the car comes from how much grip your tyres have during breaking. Are you struggling to get the ABS to come on? If not then you need more grip not bigger brakes which means wider/softer tyres.
If you are struggling to get the ABS to come on, meaning you are not maximising the available grip, then you need more braking force. Bigger calipers/discs/pads means more contact area between disc/pad but it also means you are spreading out the force on the pads so you get less pressure per unit area. So you need more brake fluid pressure so push harder
or add more assist to the brake servo. That or you need pads that produce more friction for the same force applied. The main advantage of bigger calipers/pads/discs is that they can dissipate more heat. This means you can pull more kinetic energy from the car, but you need to generate the fluid pressure in order to apply the braking force and extract the kinetic energy. If you apply enough force you can generate big brake force from small brakes but they will get very hot and eventually overheat.
In short if you aren't changing how much fluid pressure your generating then the benefit of bigger callipers etc is negligible assuming the brakes aren't overheating.
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