but wouldn't it be more beneficial to run the ducts to the brake calipers, since this is where fade originates?
Wrongo
the fade comes from the rotors exheading their heat absorbtion capacities- the calipers will continue to squeeze no matter the rotor temp as long as the fluid within does not boil.
you have to cool the rotor to keep it within its "range" to be able to brake. If you have ever gone on a Very fast drive anywhere you can get out and see you r rotors glowing reddish and possibly smoking. if you have looked at other iron disk racing cars you will see them at white hot frequently!