[RedDevil]

That´s exhaust pressure you´re talking about. I was reffering to the heat part.

Even so, most of the time you do not want exhaust pressure. Performance goes up when exhaust pressure gets down. At WOT there is some residual pressure at the bottom of the power stroke that could be of use, but not at light load.

That´s why (exhaust) turbo´s (compressing intake air) work so well: they do a lot at WOT and nothing much at light load, just what you need.

At light load running the A/C on exhaust pressure means the pressure differential needs to be high enough to spin the turbo as the volume is low. Then it will work against the engine, requiring much more load.
At WOT the pressure differential can be lower, and tapping that would not matter much as there is an overage. Even so, you are not at WOT for nothing; you would not like to do anything that reduces power then.
A turbo that uses a high pressure difference at low speed and can still function efficiently (having relatively low drag) at high speed would be a golden nugget.