i've been reading up that this all depends on the brake system...it is easier to focus more air through a system that creates a vacuum (reading up on f1 brakes the rotors are designed to create a vacuum like a compressor wheel on a turbo charger...by creating a vacuum its ducting runs through a system designed to pull air through decreasing braking temps)...basically a ram air effect with a compressor forcing even more air through it

on a normal economy car with stock brakes the dust shield impedes air flow (and heats up retaining heat in the system) even with ducting b/c you are trying to force air into a system that does not dispell it easily (no vacuum)...removing the shield and running a duct to the rotor seems to be the best method