I have noticed, at normal driving speeds, that the a/c does absorb some engine power -- not a whole lot, but enough to be noticeable. However, at some higher RPM (don't know the exact value, but seems to be over 4.5 or 5K RPM), the a/c compressor cuts off and you get full power. The side-effect is that, if you keep the RPMs up, the inside of the car will get pretty warm.