Don't swap out your air filter for a K&N...
K&N filters claim to add more horsepower by letting in more air to your engine... This IS true
But how? You ask yourself do they let in more air than other filters. Well, the answer is very simple... They let in more air because they are more porous thus while they let in more air they also let in twice as much dust.
Now, I don't think you race your truck and that you need the 1/100's worth of a second differance to beat someone. If you think you need it just remember that you're going to ruin your truck in the long run.
My father works for NAPA and I've gone to some of this company demonstrations where they show the pro's and con's of all competitive filters (air and oil) and K&N has given the most power, but at a very high cost (your engine).
Just my .02