Fact no.1 - Stock air intakes are restrictive.
Fact no.2 - Paper air filters are about as restrictive as you can get.
Its not mickey mouse physics at all. Open up say a 16v air box for example on a mk2. It is very restrictive to air flow.
Fact no.3 - Paper air filter do not flow as well as k&n filters(cotton fibre i believe)
Fact no.4 - Better air flow means better engine responsive.
Try drilling the wing side of the airbox on a 16v and remove the trumpet, then add a k&n panel filter.
It makes a considerable improvement to the driveability of the car, in general much better throttle reponse and improved mid range torque. It should be noted however that top end performance was not really noticeably improved.
I should know, i have owned 2 16v's.
I don't dispute that doing these mods will have a huge impact, they won't, that is obvious.
However its very basic physics, people that try to claim its false, well need to go back to school really
