Just reflash of the ecu and these are the numbers Jimmy LaRocca put out on a stock 03 Mach with just K&N. Jim said he saw no real performance out of the K&N. Jim also states the he believes the earlier 03's have a softer tune, and the later 03's and 04's have a stronger tune.
http://www.mach1registry.org/forums/showthread.php?s=&postid=81301#post81301
http://www.mach1registry.org/forums/showthread.php?s=&postid=81301#post81301