I thought fan restriction operated on the inverse square law, i.e. twice the distance - 1/4 the restriction.

Thanks for the test data. I've been using the 3-piece plastic on the basis that they would do better over time in a dusty environment, IOW they wouldn't clog up enough to affect performance as quickly as the other designs and would be more effective (how long until a noticeable buildup inside the machine - another aspect of filters available for testing ;-) . Also be aware that the plastic, 3-piece jobs can come with two different porosities of filter material (see the Mouser catalog).
. Perhaps when you get some time you could investigate the clogging rate aspect of filters, Vapor.

Here is a random datum for you: the metal "mesh" on the front of the fan holder in the Silverstone Kublai cases drops the fan RPM about 100. My "mesh" grows an intolerable skin of dust in a week or so. I don't mind that when I replace the "mesh" front with one of the plastic filters as it is supposed to collect dust...

.bh.