[brucepick]

To look at it from a slightly different angle -

Forget the O2 sensor(s) for a moment. Think about the fixed-diameter intake path. CAI helps because the denser air allows more oxygen molecules through that path. WAI will allow fewer. But then we're not looking for max HP.

So likely benefits are reduced intake turbulence and aero resistance in that path due to less dense air. I think that's also called pumping losses. The gas miser's engine is usually running at low throttle so being able to run at slightly wider throttle for the same amount of O2 gives less pumping losses.

Also I think WAI gives easier/more thorough combustion due to the warmer temp.

Back to the O2 sensor. The feedback loop will do a good job ensuring that the actual air/fuel ratio stays at the designed value. So it really doesn't matter too much what temp air you put in, at least as far as the O2 sensor is concerned. Intake Air Mass Sensors are a different story. I think different types respond to air temp changes differently. But since the O2 sensor has the last word in managing the air/fuel ratio, I don't think it matters much unless you have the old Bosch/Volvo type intake sensor that is damaged by hot air.