^ this is very true! Also explains why early 4.6l fords harmonic dampeners started at 5lbs weight and grew to over 12lbs later in the years.
Im late to the thread and i dont know why, but ill include my two sents.
Underdrives or underpulleys....whatever you want call them are really not an upgrade due to ford getting the speeds and feeds of the pumps right. However, the pulleys are heavy! And we know heavy rotation mass loads the engine. The engine requires power to spin them and the heavier they are the more power required. Lighter weight pulleys leave the power needed strictly to drive the accessory, but this is getting down to the science of freeing up horsepower.
1-2hp could mean a few tenths in very competitive drag racing. It could mean the edge needed to pull away out of a turn quicker on the circuit tracks.