Coefficient of Friction is best use to describe the braking efficiency. For a brake to work efficiently we have to look at it from a bigger picture. A few things that governs the efficiency of a brake.
Types of rotors & size, pads, hose, fluid, caliper size and pump force...etc...etc..
As discussed by all the guys, Brake uses friction to stop a moving object and hydraulic pressure with the help of vacuum servo is the medium to create friction in a brake system. The higher the pressure, the higher the friction. The higher the friction then it will create more heat, choosing the right temperature of pads to use there for its important as some high performance pads will have high temp rating but it will not work on normal daily drive because the pads material needs to get up to operating temperature to work efficiently.
Having said the above, for a brake to work efficiently we also need to know how to transfer the brake pressure efficiently. Does the brake pistons get all the brake pressure it supposed to get? Brake pressure can be lost in the transfer process due to heat and expansion.
Then, there's the road condition, tire's condition...which also contribute to braking efficiency. Even with a 4 or 6 pot...you still won't have a good braking efficiency on ice is you are not using winter tires...
So choose your pads & rotors depending on your usage...
Just my 2 cents