We asked our buddy Dan Bills at Finish Line Performance if he has ever made back-to-back chassis dyno pulls with the air conditioning in the car on and then off to measure the power difference... here's his response:

"Kevin,
 
  Yes I have, depends on the car and type of system, usually about 10 hp. Most of the new cars shut off the compressor over 4000 RPM at wide open throttle so the difference is nil."

I remember talking with an aftermarket / oem parts supplier about having the alternator on a clutch as well, freeing up power under acceleration.     That was 6 years ago, and never saw it materialize.   Could be another free 5-6 HP under wide open throttle.