There is always a perpetual argument whether any so-called high performance air filter, either oiled foam or oiled cotton gauze like the K&N, filters the air as well as a stock type filter. I think the truth is mixed. On some cars the K&N filters as well. On others, not so well. Doing the actual testing with mixed particle sizes is an expensive undertaking.
If there is any performance gain of increased air flow resulting in higher horsepower, that would be at only high rpm and wide open throttle. At less than floorboarded the throttle is doing it's job of, er..., throttling the intake air. At less than high rpm the engine isn't pumping enough air to make any difference. In a turbocharged engine the turbo needs very hot, very high volume of exhaust gases to spin up to its max and pump in the max air.
Can we all agree that there is no fuel economy gain in any modern computer controlled engine (since the '80's) with a so-called free flowing air filter? The computer controls balance the amount of fuel to match the amount of air. In a carburetted engine a clogged air filter could result in a rich mixture, but that was way back then.
