Well from what I know, the engagement of the AC compressor puts more drain on the pulleys, which is why the car loses power because it has to drive another pulley.
If I am going to race around, and I want maximum Accelleration, AC and anything else that can draw from the engine is turned off if it can be.
My jeep definately has less power when the AC is on.
My benz I can't really tell the difference, but then again I drove the jeep for 7 years, and I have had the benz for 7 months.
Alon