I have a VERY hard time believing what you say!!I can't recommend enough for anyone to install a performance filter in the stock air box, if you don't plan on making more upgrades any time soon. You do get a little power back but more so you get much better fuel economy.
You may not notice a difference at first especially if it's on a smaller low powered engine. But if you do want to see how much it changes, drive on the new filter for a week or so then put the old factory one back it. It's like pulling a car behind you. You will see the difference.