I swear by K&N filters. Not only are they cheaper to maintain over the long run, they really do help breathing. Personal belief is those long convoluted air intakes are mostly silencers, nothing more.

Where you should notice performance boost most is in mid-upper RPM range, when engine is really starting to pull in a lot of air. (Past, best experience is with a '97 Jeep w/ 4-cylinder. Could barely get to redline with stock, but removing airbox and adding cone type filter allowed it to rev much more freely in higher RPM's)

In two weeks I am deleting the airbox on my '00 S70 low pressure model, and installing the K&N "cone" filter. As I have Firebird Raceway right down the street, we can legally drag race every Sat. night for $10.00US. I'm geeky enough to baseline mine, and then retest with new parts, so will let you know what I get.
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2000 S70 Turbo - and loving life!