There is research on-line relating to this stuff. I've cited two, below. Ultrasonic 'perception' is established, but its application to normal hi-fi audio is vague and tenuous. I'd say it has no application at all. My admittedly lay investigations show that 'perceptions' of frequencies up to at least 100 KHz are possible under very specialized (direct bone conduction) laboratory conditions. However, perceptions are not 'hearing' in the normal sense of the word.
A) Although testing of higher frequencies with bone-conduction is rare and unstandardized, Corso [Pitch Discrimination at High Frequencies by Air and Bone Conduction, American Journal of Psychology, V. 78, 1965.] measured bone-conduction thresholds at higher frequencies with procedures similar the ISO statndards, but did not have any masking stimuli. Furthermore, bone-conduction may facilitate sensitivity to sounds even higher in frequency than can be detected through air-conduction, possibly up to 95KHz. [Thresholds of Audibility for Bone-Conduction Headsets, Walker and Stanley, Sonification Lab, School of Psychology, Georgia Institute of Technology: Proceedings of ICAD 05-Eleventh Meeting of the Internation Conference on Auditory Display, July 2005.]
B) Humans can detect ultrasound up to at least 100 KHz, but perception generally requires direct contact of the source with the body. Ultrasound sets the brain into forced vibration, and it is the brain oscillation that is detected on the base of the cochlea in normal hearing individuals. With hearing loss, greater ultrasonic energy is needed to spread the displacement on the basilar membrane toward the region of intact hair cells. Ultrasonic pitch is not related to the stimulating frequency, but rather to the remaining high audio frequency ability of the listener. [Ultrasonic Hearing in Humans: Aplications for Tinnitus Treatment, Lenhardt, Department of Otolaryngology, Virginia Commonwealth University, International Tinnitus Journal, Vol 9 No2, 2003.]
It must be noted that what is described is an in-band perception of an out of band vibratory stimuli 'bleeding' into the normal hearing range. It has nothing to do with ultrasound high frequencies somehow extending our normal hearing to outer limit ranges, or making what is heard within the normal range of hearing 'better'. It appears that 'ultrasonic' mechanisms may show some benefit within a medical context, for example in the treatment of tinnitus, but as far as anyone benefiting from ultrasonic frequencies as they might relate to their hi-fi system? Well... that is not a conclusion that can be supported by these studies. At least as I understand them. Again, if someone else has more information, I'm happy to learn from them.