True, a "low restriction" air filter would provide little or no gains in power on a naturally aspirated diesel... *unless* the injection pump were calibrated to anticipate the higher quantity of air.

A turbodiesel *would* show a gain in power automatically. The turbocharger would not have to work as hard pull in air--thus the turbine wouldn't need as much energy from the exhaust, etc.

I have heard the 2 schools of thought regarding the filtration ability of K&N filters... a lot of it sounds like speculation and rumor.

Personally, I have used K&N filters on some gasoline engined cars and noted that the throttle body and intake plenum remain very clean over a long period of time--cleaner than with the stock paper element! I believe the oil in the filter element is the key.
-Joe