My wife bought me nautical air horns as a birthday present you can probably see in my pic here mounted over the driver's window. They're absolutely awesome (as in, 'make me giggle' just thinking of them), but honestly they're more about fun than practicality, at least any other place than the freeway. They're simply too loud for most situations. Depending on where you're at, firing them off may even be considered illegal.
If I were you I'd skip the air, and look for a loud electric horn that mounts under the hood / bumper (maybe look for models popular in the first-responder sector). Adding an electric circuit, if necessary, is (to me) easier than adding an air circuit. Depending on the amp draw and your horn wiring, it may just be a matter of swapping out new for old. As a matter of fact, doing the exact same thing is on my 'to-do' list.
Most people in a time-critical situation that requires a horn will instinctively push on the center of the steering wheel. I think that's another good argument for upgrading your stock horn while keeping the actuation the same.