I upgraded my horn to twin FIAMM elestrostatics. Very loud. Not quite as loud as air horns, but they are instantaneous.

I do suggest using a relay because a louder horn requires more power, and the lead to the stock horn is wimpy as.

What I did is mount a relay where the stock horn was and use the wire to the original horn to trigger the relay. Then I ran a +'ve lead (RED) directly from the battery (with an inline fuse) to one side of the relay and a switched lead (blue) from the other side to each of the horns. The relay and both horns are grounded to the bolt that holds them.

The horns are mounted on either side of the grill using the supplied brackets and bolts that I assume hold the front bar on (10mm).

They are a drastic improvement over the stock horn with no drilling, cutting, or any other interference that would stop me returning the car to stock (as if I'd ever want to).

I had to angle one horn back a bit to fit my new Aus Intake Extension but it's not big deal.