I think the ultimate theory is that the underdrive / aluminum pulleys, whether on our application or any other make/model where light weight pulleys are available, is that they don't actually ADD horsepower. What they do is to decrease the drivetrain LOSS ratio. The engine can rev faster/easier, and the power generated by the motor has less motor/weight to turn in essence so more of the power it generates actually makes it through to the wheels to be measured by a wheel dyno. It's the same concept as light weight wheels, flywheels, cf driveshafts, etc. Less weight in the driveline.
And as far as the accessories actually taking power from the car, well, if you're ever had the ac compressor or alternator off of a car, or teh belts off, those things take ZERO effort to spin. You just flick them and they turn and turn and turn.... So for those, just turn the AC off and there's your power gain regardless of WHAT pulley you have.
That's my "i got two business degrees" theory
EricB