A mousefart horn is barely loud enough to be heard over an engine at tickover..a là most standard bike horns...car makers usually do better at this,so you can sit in a traffic jam and bleat at bikes filtering past,lol...
I put the full sound bomb on the GS after a guy nearly took me out on the M6. I have to say its the best things I've done and will be upgrading future bikes with the same.
My BMW RT has twin wind-tone horns, similar to most cars now; those & the fleuro go-faster stipes tend to get me noticed & make people behave! Even if it`s only for a moment before the second glance, it`s enough to keep me alive so far.
I have reservations about loud horns, for me a loud horn would startle and possbly cause a motorist to react without thinking or freeze in the middle of the road. I think your better with a constant presence like a louder exhaust or bright side lights (maybe of slightly different hue).
OP - at the NEC bike show last Nov, R&G stand had 3 horns wired up to doorbell buttons on the counter. One was a stock jap horn you'll find on every Honda, Yam, Kwak or Suzy and probably loads more. Next to it were Denali sound bomb and sound bomb mini. Well of course I pushed the buttons. Who wouldn't? 15 minute later I'm stuffing a sound bomb mini into my backpack - my ears still ringing. Chose the mini because a much easier fit - straight swap with no fettling or wiring needed. HUGE improvement!