I ran across a replacement horn option for my RT on an RT-oriented forum. It's plug and play as it's actually a BMW car horn. It doesn't create any canbus issues on the RT and I'm curious if anyone has tried it on the XR.
The horn is from an early 2000s 5 series. There are two manufacturers. The one to get is the Clarton horn (low tone) as the other is aftermarket and can sometimes cause some issues.
I would have tried it myself, but the XR horn seems a little harder to get to than that of the RT.
The part number I used to order it is: 61338381271 and it runs about $30 in the US.
It's not a ton louder, but it definitely sounds more imposing than the typical meep meep of the factory XR/RT/GS horn and is not startling like some of the air horns tend to be.