Its important to understand that more pistons does not mean more clamping (stopping) force. If you want better stopping power, you need better tires, if you want longer lasting brakes with more even pad wear / better heat retention, you need a bigger brake kit.
The only time a bigger brake kit will add more stopping power, is when the rotor is larger, because then you're physically putting more torque on the hub, however, it again comes back to your tires.
here's a good article you can read on the BBK subject.
autotrackdaymonthly.com - Brake Upgrades for Track Day Use