The stock cooling with stock (or GiroDisc) rotors is very good, under most conditions at most tracks. The exception is extended trail braking after a hard braking zone (e.g. VIR or Road America). The symptom is grooving of the front rotors, caused by hot spots. The lighter, smaller, AP rotors have less inherent heat capacity, and benefit more from directed cooling. I believe for the stock set-up there will be a rotor and pad life benefit for those who are hard on brakes at brake hungry tracks with the addition of directed brake ducts. I view them as mandatory for race/hard track use with the AP set up.