3" ducts, driven on street for a few months and had one track day on them. Drove same track in April/May without them on the same pads and tires and brakes held up a lot better this time. Drove one full session full out without any fade or issues (stock pads, rotors....stock car actually plus ducts).
These ducts are a direct run to the rotor hat from the fascia, so the velocity of the air is as good as I can get it. I actually took a leaf blower and had my wife hold it in front of the fascia. The duct was blowing pretty hard, felt like there was no loss in velocity inlet to outlet.
Finally, by not using a inlet mounted to knuckle I dont' have to worry about all the other articulation issues due to steering but the air still hits the hat pretty well.