The 300Hz Stebel electric/air horn is now in and going, and working nicely. I have not yet had need to use it on the road, but the two people that have heard it working along with the two OEM horns report being startled and surprised by its impact.
I can now personally verify that while the horn has a relatively rapid response for an air horn, that the two electric OEM horns respond noticeably more immediately, and the air-horn takes a good fraction of a second longer to build up to full output. As other forums have indicated, this actually makes for a more usable arrangement, with progressively longer horn button depression times increasing the seriousness of the emitted horn sound, but the OEM horns still providing an almost immediate response to start the warning ASAP.
In terms of installation, I purchased the 300Hz horn with an optional Horn Wiring Harness kit, which made the whole process more straight forward.
Working out where to put the 30A relay that comes with the horn, and where to put the horn itself, took as much time as actually putting them in. I did manage to find existing empty mounting holes for both the relay and the horn, so no drilling was required.
The relay is mounted inside the engine bay, right of and below the fuse box, while the new horn is mounted just left and below the hood release lever, just below the black cover plate. With these locations, the lengths of the cables in the Horn Wiring Harness Kit were just right, reaching the battery terminals, and the existing top right OEM horn nicely.
The only bit of cutting I had to do was underneath the black cover plate, left of the hood release, to prevent some of the black cover moulding from protruding down into the new horn. I made a vertical cut of about 3cm and then folded the flap back horizontally, which now rests on top of the horn. None of this is very visible from either above or outside. Also, the flap now provides some failsafe downwards pressure on the top of the horn compressor - if the single horn mounting bolt ever becomes loose, the flap will help prevent the horn from popping up off the mounting bolt and falling around free.
While I am waiting to see how the response of other road uses is altered by the addition of the 300Hz horn, at this stage I am pleased with the outcome. This is one of those mods that doesn't cost much, but makes a disproportionatly large improvement.