Natalie,
If you had only asked earlier.. I would have shared that information... I did not realize others had been keeping it from you...
From what I have read.. under full heat load and at speed ( which increases the loss from the hot air going over the car and expecially the windows ) an air conditioner can pull as many as 8 hp from the engine...
Then add that with over 100 degrees in some of these examples... that the air is thinner due to the heat... (thus the engine has less air stuffed into the chamber)
And I am surprised that 5% is all the drop that is manifested. I say it is well worth it for the comfort of Air Conditioning.....

"The mileage in all the cars drops a few percent with the AC on " this only applies on the new wind tunnel cars if you keep the windows rolled up.. as that is how they are built to get such good mpg in the first place....