I had an air-horn setup on my car, It cost me about $150, and honestly.. it was a waste of money.
Air horns take time for the compressor to start turning to blow the air. This means that from the time you push the button to the time the sound comes is about .3 seconds, which doesnt sound like much, but in an emergency situation, trust me, it matters.
Also, since the pressure is not coming out as a burst, but instead as a build up, it doesnt have the instant presence that you want for a loud horn, its kind of like an old siren winding up. (alot faster, but still not as startling as a full train-horn).
In truth, you would be much better off swaping the stock horns for a set of hella supertones. They are surprisingly loud, and they are super easy to install.