First of all, get a fordchip and it will make a difference. yes a diablo chip will give you about 5 hp just becuase of the extra timing you add with it if nothing else. But a fordchip is a little more expensive but is tuned professionally and will give you greater increases for your buck...www.fordchip.com
I got it so I could rev the motor at a stop (factory limiter for 03 is 4,500rpm) and get rid of the speed limiter. I can now top out at 148, much better than 120.
I installed the diablo chip along with my other mods, and i can feel a little difference from when its on performance and stock. If your thinking about gettin some other major mods itll be worth it. I like the chip though, even since i dont have major mods.