I think I read a "study" done on just this question some many years ago. The findings were somewhat interesting: they found that the air moving over the curve of the base tended to increase velocity to some degree, and that the small restriction of a low cover increased that effect. At some point, though, things went the other way; either from too much restriction, such as too narrow of a filter that brought the lid too far down and seriously restricted air flow, or a taller filter element that raised the lid high enough that the Bernoulli effect was diminished or eliminated altogether, and turbulence took it's place.

Now there is some fodder for a serious discussion; go to it gents.

How's that for digging into the dusty, web choked crannies of my addled brain?