here is another option I think is a good idea.......
I've got the Stebel Nautilus on my '05 R1200GS. It certainly helps my on my commute in NYC.
Excel Cycle Werkes sells a mounting kit with everything you'd need for an R-bike.
webBikeWorld has a good selection of articles on MC horns, and comparisons, here.
Just make sure you plug in the relay properly. I did not and blew mine out after less than a year. Of course, the $14 relay was the last thing I checked, after buying dual-Fiamms and a trick wiring kit from Eastern Beaver. I keep the relay as a reminder to check the simple (and cheap!) parts first.
Wolo "Bad Boy" dual tone air horn with relay - $40 at Harbor Freight.
Cheap, easy, loud.
The '03 RT came with two Bosch horns, high and low. Very loud and effective. No sense in changing.
My 83 R100RS has a pair of Fiamm's. They're loud enough to wake the dead. They were on it when I got it. Are they stock?
It allows you to keep both horns with a single horn button. Short beeps use the stock horn but hold the button down for the air horn.
1983 R100RT hacked w/Cozy Rocket My blog
2012 Ural Patrol