Okay, here's what I've learned:
1. The maximum SPL on any Fiamm non-air (I mean, "electric") aftermarket horn for cars is 130 db at 4". That's the SPL of the AM-80, which is the Focus stock horn. It's the SPL of the Highway Blaster or whatever it's called. Heck, it's the SPL of basically all of Fiamm's electric horns for cars.
2. The maximum SPL for the airhorns is an order of magnitude louder. It's maybe 118 or 122 db at 3 FEET. I think that's probably a very significant real-world difference!
3. Some owners complain that the Fiamm automotive air horn kits tend to start to die after two years or so. The compressors seem to slowly lose their compression. Here are a couple quotes from the Miata list, both from reviews of the "Riviera" kit DHF has installed:
REVIEWER #1: "Fiamm Riveria Two Trumpet Air Horns...I am installing my third set in less than 1 year. I love the sound of the horns, but for some reason, they don't last. I am not on my horn very long, but only after a couple of months one of the horns wares out and needs to be replaced. Much better sound that stock meep, meep horn and worth the money, even if they are disposable."
REVIEWER #2: "A couple of years ago, I got the FIAMM [Riviera] dual air horns as a present. After about 18 months, they began to get weaker; by the time I replaced them, they were sounding like a weak goose. I tried various testing to see if it was an air leak, and determined that the compressor simply wasn't up to snuff anymore.."
4. So clearly there'd be a market for a much louder electric horn! Why isn't there one? Perhaps maximum SPL on stock horns is limited by the dept. of transportation...
One solution would be to simply splice in one more AM-80 aftermarket horn.
DHF, do you think this can be done without a relay or anything -- just splice it into the OEM AM-80's leads? If so, that should yield a total SPL fairly similar to the air horns.
It would be nice to get the two AM-80's in different frequencies, to get a harmonizing sound.
Is the OEM AM-80 the "low" or "high" tone? (the AM-80 comes in two versions -- 405 hz vs. 485 hz). See: http://www.stealth316.com/2-fiammhorn.htm