Since water goes right through when cleaning, I would expect nothing at all would happen if it gets wet. It would act like the metal mesh filters used on hot rods of long ago. They used a few drops of oil to increase filtering. Cotton was mentioned as a possible element. I considered that but abandoned it since cotton is so absorbent.
So I guess what the problem was the stock filter was getting saturated with water which caused little air flow. And that the electrostatic filter allows water to pass thereby not impeding air flow.