I found the K&N filter improved performance specifically when my air conditioner was on. From day 1 the car would be sluggish with the air on. After putting the filter in, taking off from the line was improved, my gas mileage improved slightly and high speed accelration was better.
I have done a lot of reading on filters before I put it in, some say they do nothing others say they do. I believe it likely depends on your situation (i.e. the way you drive, were you live and the type of car) as to how much of a difference you will see.