Here a dude tests an underdrive pulley. Although he is careful to use the same machine before/after, we don't learn anything about the ambient air temp or the condition of the car. And I am very skeptical that an underdrive pulley can make this much difference:
This video is a nice dip into car culture on the subcontinent. Not realky adyno test of a mod, but more of a dyno test of loss of power due to age. These guys think they have shown there was almost no loss of power in 12 years. However, the change in HP could just be an artifact of the test itself: