I don't understand why the mileage would stay the same. An underdrive pulley improves mileage, so why wouldn't removing A/C from the belt? By removing the A/C pulley, there's less drag on the belt... that's good isn't it? I guess I'm missing something.
When the A/C isn't engaged, its just a pulley that rotates freely. The pulley isn't driving anything and so the drag is minimal.
I would have thought even with the A/C disengaged there would still be resistance in the pulley just by the mere fact that it isn't an idle pulley. The pulley is still connected something inside the A/C unit.