I always keep it on manual control so its always going to the screen and vents, and with the aircon 'on', you can hear the compressor clutch disengaging when the foot goes down
then re-engages when you lift off.
Never said your point re aircon using more fuel didn't stand, was just disagreeing with it making a big difference over a 0-100 sprint. It will have more drag on the engine as you first pull away and get up to full throttle (or whatever the setpoint of engine load % it needs), after that it would stay disengaged until you lift off.
Felt it going down the strip at crail, sat at the line in cool aircon'd air, foot down and you can feel the difference in the interior temp by the time you get to the end of the quarter, then it cools again on the slower drive back to the strip lol.