When GM measured our LT1 camaros at 275, was that with the a/c on or off? Does our stock Air conditioning rob us of any significant hp?

 

At WOT the AC compressor disengages so you don't lose power... just as if the AC was turned off. If that answers your question.

If that doesn't answer your question then, yes, the AC system would have been on the motor when GM rated the motor at 275. There is 'some' hp loss but it's pretty minor. <5 HP.

The major downfall off the AC system is the weight associated with it... which obviously you don't care about because of those heavy speakers you have in your car.