Thing is, there's no guarantee that your mileage will improve. There are just too many variables.
On another note, the K&N filter won't do anything positive for you except for the clean/reuse factor and it can have an adverse effect. The filter oil can foul the MAF, and it does not filter as well as a stock paper filter. If you want a quality reusable filter, look at the Amsoil filters. They are dry filters that you clean with a vacuum cleaner.
2003 F-150 XL RCSB 2wd 4.2 M5OD 3.55 LS