I know this is going to sound rude, but...
Why are so many people having such a difficult time understanding this concept? Assumming you start with a stoich. mix, the more air you add without additional fuel leans out the a/f mix. The 05 Mustangs come stock running on the rich side, so you can safely add a decent amount of airflow without thinning out the a/f mix so much that you experience immediate, noticeable problems while gaining an immediate benefit in tq and hp.
However, the better the cai/induction system is designed, the more air will get into the engine, and hence the more the a/f mix will be thinned (i.e. leaned) out.
If you don't compensate for this by adding additional fuel (and then adjusting the timing, etc), then you'll only gain a limited amount of extra power until the mixture becomes too lean and no longer burns efficiently.
If you can add an induction kit without retuning the engine, then, engineering-wise, that means the induction system is only increasing the airflow enough to compensate for the already-rich a/f mix from the factory. Any increase in the airflow beyond that point will continue to further lean-out the a/f mixture, with the ultimate result of your CEL idiot light going on.
This is how some of these kits require a re-tune, and others do not. The C&L intake and the Steeda Intake (as two examples) increase the airflow so much so that a/f mixture is leaned out too much, and the mix is no longer close to stoicheometric.
If you're looking for the maximum bang-for-buck with a n/a engine, then you want to increase the airflow as much as possible, while keeping it as dense and cold as possible, and then adjust your fuel add and ignition variables accordingly.
Maybe the moderators here can make some kind of sticky about this. But get somebody from Ford Racing to write it, because I know people are going to continue to argue about this mechanically basic concept.