Number of pistons have absolutely nothing to do with maximum stopping power. Max stopping power is limited by traction of the tires. Any modern brake setup will be limited by abs/friction and the type of rubber they are on. As well as the mass of the vehicle and velocity that vehicle is going.

Increase number of pistons apply the brake more evenly so over more constant brake action such as on a track, the rotors will dissipate heat more evenly and will not get "hot spots" from uneven application of the pads by the calipers.

More calipers is not better for anything other than more consistent wear and tear or track use where brakes heat up signifigantly and consistent spread of heat is better than isolated heat dissipation

 

Ahh i see, now i got something to tell mi buddy

 

Whoops

 

While on the subject of brake pistons, can someone tell me do I just press in the piston or do I need one those special tools to push and rotate at the same time?
I have an E39 but plan on doing my brother's E60.

 

Just press it in without rotating.