Swapped my horn out today. I went for Zanderk's solution in the end, coz I like easy and coz I couldn't bear the thought of something not new and shiny on my bike
From my point of view the new horn sounds much better, but I don't think it's actually any louder. It's just a much nicer tone. In fact I just checked specs on the web and it looks like both the stock horn (Clarton E9) and the one I bought (Fiamm Am80SL) are rated the same at 107 db.
While I was at it I looked up the Denali soundbomb mini, which is a fair bit louder at 113 db, and the Denali soundbomb which must be loud as balls at 120 db!
Maybe useful info for others considering this mod.