What i noticed when i put the K&N wioth the stock air box, was better gas millage (slightly) and not really any noticable power....The air box is really restrictive, i know you want to keep it stock but sometimes stock and power don;t go hand in hand.
If you want to keep it stock, go with the K&N drop in, but be carful with reverse filters, im not sure about k#n, but i knw KKM has problems imploding...(you may not have the problem with the stock airbox). But if eventually your goign to go performance, i reconmend getting a MAF adapter and creating an open filter eliment, removing the stock air box.
With the open filter, you should see and maybe feel the throttle response.
Roz
