Under part-throttle conditions, the breathing must be restricted in order to keep the engine from running away with excess power. That is what the throttle valve does. Helping the motor breathe doesn't do any good whatsoever until you have already set the throttle wide open, i.e. full power. Most of us don't drive in that condition very often, especially when pushing for fuel economy. Part throttle efficiency is hampered by pumping loss, and warm air helps reduce that loss a bit. Here is one thread on the subject.. There should be several others floating around here.