Not really a scientific test, either.
A stock air filter will flow far more air than your engine can use. Since this is a proven fact, that means there is no way there can be a gain by simply changing an air filter. No HP, no mileage. And even if there was...is .5-1mpg that people claim worth allowing dirt into your engine?
Plus, a stock filter is FAR superior at filtration.
K&N's are notorious for letting dirt into engines.
Ask any reputable engine builder and he'll tell you that he can tell by the wear that an engine had a K&N or other oiled filter.
Cost...A K&N is 40+bucks. Precharger to go on it is another 20..and you need this to even get close to stock filtration ability.
Plus the oil to "recharge" the filter. So you'll have a solid 75 bucks in a K&N setup....how many stock filters that do a better job can you buy for that?
You'll have lots of fanboys who believe the K&N hype come in here and argue against me, but isn't that always the case.....