CAI's are over rated, especially when it comes to the Mustang. The stock air box flows as good as 90% of the CAI's being sold today for the mustang. Now if you want looks or sound, then by all means buy a CAI. But don't buy one and expect the claimed 10-12 HP gains with a CAI alone. If you buy a free flowing exhaust, and a tuner (tuner is the best bet), then you'll see HP and TQ gains.
Your current air box flows just fine. It can be made to provide the sound that a CAI produces by simply removing the muffler off it's side, and fiberglassing the hole shut, then repaint.
But be quiet, be very quiet. The folks that make CAI's or the ones that sell them don't want you to know this. They sell kajillions of these to late model Mustang owners to those who don't know.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.