That theory would make sense, if they didn't put the rubber down inside the air filter about a third of the way. That's why some guys just cut the the part that sits down inside the air filter. As the part is engineered, it sure looks like to me it's restricting air flow.
FWIW, when I took the snorkel off 2 years ago, and the dealer re-jetted with off-road pipes, they told me it was the biggest jet size increase(to 135 mains) they had done to that point on a Speedmaster or America. It wasn't dyoned, but they did tune with an air/fuel ratio tester.
