I replaced the Elise stock horn with a set of two FIAMM horns. The packaging is pretty bland, stating "Replacement Horn" "125 Decibels", and the size and form factor is pretty much like the stock horn.
In fact, after removing the stock horn, I wasn't convinced this would be much of an upgrade. Our stock horn is a bit beefier than one of these two FIAMMs, not too shabby.
As others have noted, removing the mounting nut behind the stock horn is tricky -- there's a lot of stuff in the way and no easy way to get a wrench to the nut. I cheated and just twisted the horn to loosen it, careful not to pinch the wires. That released it enough that I could see to the wires, push the spades straight off by poking with with a flathead screwdriver, and spin the horn all the way off. Much easier than dismantling the wheel well and right hand oil cooler.
The FIAMM horns come with a snap-on wire connector and pigtails, which makes wiring, uh, a snap. I made up a harness with two flat spades (for the stock connectors), and both plug connectors. I added a bit of zip cord in between to reach across the car.
There is an existing (oil cooler?) bracket in the cavity with bolts that extend quite a bit through the spot-welded nuts. The FIAMM horns included a nut and lockwasher that made a perfect fit on these bolts, so I took that as a sign!
The stock mounting location is probably "better" in that it's more direct, but without better access, I wasn't going to try to mount a new nut back there.
I don't know how directional these horns are, but Forward toward the grill seemed sensible:
I was going to do the same thing on the left side, but kept getting interference from the Cobra siren unit. The FIAMM kit also includes a right-angle mounting bracket, which proved ideal for sharing the siren mount.
I removed the washer from the existing nylock nut, slipped my bracket in its place, and aimed the horn. Perfect fit.
All the nuts were seasoned with blue Loctite, and the wire connections all waterproofed with solder, heatshrink, and hotglue filler. Removing the original horn was probably the hardest part. If I was doing it over again, I might just leave it in place and piggyback the new horns on with it for more volume.
The sound is loud and harsh -- just what I want from horns. Of course, even the stock horn sounded loud in my garage, so the real test will have to wait until the next SUV joins my lane. That should take about five minutes of local driving...