Has anyone ever used the K&N performance air filter (http://www.knfilters.com/filtercharger.htm)? I'm reading reviews on it and I'm getting mixed opinions on this product. A co-worker of mine purchased one for his 2006 Mustang V6 and he said he could tell a difference in acceleration and also was getting better gas mileage. I asked the person at Advance Auto and he said he couldn't tell a difference in the performance of his car or his gas. It seem to affect some cars different than others.