K&N FILTERS
I've been on both sides of the counter, selling K&N and using K&N. It's been 10 years since I sold them, but I still use their oil and air filters today on my autos and motorcycles. I personally have never had a problem with dirt on the other side of the filter. I ran their FIPK (Fuel Injection Performance Kit) on my 5.0L Mustang for many years without ever seeing any dirt in the mass air meter or throttlebody that would have gotten by the filter.
Noisier? With just the panel replacement filter, no. With the open FIPK, yes. Kind of like an open element air filter on a muscle car with the Holley wide open. If your a hot rodder at heart you'll like the sound. From inside I could only hear it under WOT on my Mustang.
I run Amsoil oil in my autos and motorcycles, and their oil filters are not bad at all (they are made by a major oil filter manufacturer for them), but I don't like the foam air filters. I've seen dirt get by them, and by the time you get the foam dense enough to trap the dirt you lose flow. Just my observations.
Fuel mileage? I've never noticed a real gain just by changing to a high flow air filter. A recent test I saw in an import car magazine tested the high flow air filters set-up from several manufacturers and they experienced about a 2% gain in HP across the board by all of them. So the HP gain isn't great, but for a just an air filter that isn't bad.