If your only goal is to improve airflow, you are better off removing the filter entirely. You will likely damage your engine, but you will have maximum airflow in the meantime.
If your goal is to balance airflow with reasonable filtering efficiency, then the OEM filter is the best trade-off IMO.
Not much independent testing is around, but here is one link. Note that the K&N filter did not filter as well as standard paper filters, which is consistent with the improved airflow claim. In this test, the airflow improved due to the reduced pressure drop, which was measured at 0.14% improvement. That was a trade-off with the increased dirt ingested by the engine. And if you want to let 0.14% more air into the engine, you could always press down on the throttle another 0.14%.
Filtration Testing for Amsoil, K&N, Napa, Jackson Racing, Baldwin, and Mazda air filters on a Miata
Here is another link. This one is based on a GM diesel pick-up, with several filters compared to the K&N. The K&N had the lowest measured filtering efficiency, and also the second-lowest accumulative dirt capacity (before reaching a predetermined restriction level). From the summary conclusion:
Quote:
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Compared to the AC, the K&N “plugged up” nearly 3 times faster, passed 18 times more dirt and captured 37% less dirt. See the data tables for a complete summary of these comparisons.
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Full results here:
ISO 5011 Duramax Air Filter Test Report
So, before believing the manufacturer's advertising, and realizing that web poster opinions are pretty much divided, why not review those two independent reports and see if they have any credibility?