Don't forget that the air con probably pulls 2 to 4 bhp when working hard, that is a fair chunk on an 84bhp car, you would notice it in the same way that if you remapped the car and saw a 10% increase you would feel it, that would only be an 8 bhp gain.

My TVR Cerbera used to switch off the aircon if you hit full throttle...just to give that extra power....I think it was probably to stop the belts coming off but they claimed it was for full power.