The disc size has to do with heat dissipation, what DOES make you stop faster is the increased brake pad contact patch, just like tires the larger the quicker you can slow down. Also as you increase caliper and brake pad size, larger rotors are necessary to accommodate. If your tires are limiting you, you probably should buy better tires. Also by switching to the Viper calipers a wide range of better gripping brake pads become available.
