I have had several vehicles over the years with K&N filters and you should take most of their claims with a grain a salt. On a bare bone stock vehicle, you are probably not going to see much, if any, difference in HP, torque or MPG. However, if you combine the filter with other mods (such as a cold air intake, better exhaust, cam swap, turbo, etc...) you'll notice a much bigger difference. The one thing you MUST do if you do any of this is take the vehicle and get a custom tune put on it. Don't bother with buying a boxed plug-in tuner as they only change a few setting on the ECMs. A skilled person with the right program can change three times as many settings and customize the tune to your driving style and how you use your vehicle. The cost is usually cheaper also. As a bonus, if you make any other mods to the vehicle, they will usually reprogram it for free. The last three vehicles I've had tuned cost between $200-$300 per vehicle and ran and drove MUCH better than a stock tune. Two out three got 2-3 MPG better after the tune.......(the third vehicle was a 2wd truck I had souped up to blow money on and get into trouble with..

The only reason to switch to a K&N on a stock vehicle is to same money replacing air filters in the future. I live down 4 miles of dusty gravel roads and usually change my air filters every 10,000 miles. The paper filters average $8-14 per filter and a K&N is usually around $40-60 per vehicle. After 50,000 miles you have your money back. BTW, there is NO way I would go 50,000 miles on a K&N filter (as they claim) between cleanings. I usually go some where between 15-20k on mine.
Good luck!