For headphones, I think it's almost entirely irrelevant. Theoretically, if you're playing high-res music files and your DAC's low pass filter isn't chopping off everything above 20K in the first place, it could mean that the Sundara could reproduce some of the highest harmonics of instruments (there is cited research from Boyk showing that many instruments naturally produce harmonics above 20KHz, and sometimes up to 100 KHz), however, the level of those ultrasonic harmonics is extremely low, at most 2% on trumpets and usually under 1% of the total energy of the note being played. Thus, it's of generally no use.

If you want to see how well-extended the treble reproduction is on a headphone, the FR graph is a better representation even though the treble is where the rigs generally aren't that accurate, and treble perception will vary greatly with differences in the shape of the ear. The FR ranges provided by the manufacturer are practically worthless.

Since we're on the topic of ultrasonic perception, the ability to perceive "sound-like" sensations isn't limited to hearing. There's research from Lenhardt et al. that people can understand speech through bone conduction via an ultrasonic carrier wave. So even though all of the frequencies being sent to the person are ultrasonic, the person perceives speech, rather than high-frequency noise. Some hearing aids work like this. So we can't "hear" in the normal sense beyond 20KHz, but somehow our brains can glean data from ultrasonic frequencies. That's not relevant to headphones, at least none that are on the market, but it's an interesting tidbit of info.