On an 03, Id leave it alone. Might have the heads looked into, make sure the seats are all happy before ya beat on it. Might make sure the valve springs are upgraded, and I might make sure the exhaust manifolds are good, and not leaking from broken bolts.

The Diablo works great on the newer stuff, 13-17, they get in there and adjust fuel trims, timing, etc, etc, and can gain some serious HP and Tq, but unless your planning on a supercharger, or some power adder and need the PCM unlocked for a proper tune, the Diablo isn't worth the cost just as a hand held downloaded pre tune.