I found a video of the Hella Supertone horns being installed in a Subaru (apparently that's what all Subie drivers do...). They sounded like crap. A pair of standard GTI or A3 horns sound much better.
The (single) horn is actually right behind the grill, to the left of the VW emblem (left side of the car). Looks like there might be a little extra room in there to squeeze in 2. I'm thinking the grill probably comes out the same way as a MK7.5 GTI, i.e. annoying, so I haven't tried yet. I also want to see if something can be done to avoid any modifications to the wiring harness/plug. I should have some time over the holidays to dig into it a bit more.