The importance and effect to and above 20kHz...

For example, 20hkz and a 22khz interacting together would create a 2khz beat freq. Whether or not there is enough energy in this signal that could affect the hearing would be difficuilt to say

Good point. Not many people are aware that we can't even hear proper waveforms above 4 KHz....the ear's nerve bundles fire too slowly! In order to hear above 4 KHz the brain relies on two things; beat frequencies and differences in timing of what arrives at one ear versus the other.

However, very low signal amplitude because the basilar membrane does not vibrate enough to transfer energy to the inner ear at higher frequencies impies that you can't even hear beat frequencies for signals with frequency content higher than 20 Khz (or 15 Khz for most of the older folks).

Here is an interesting paper that addresses the existence of seemingly inaudible music frequencies above 20kHz and discusses the potential effects of these frequencies on the listener:

http://www.cco.caltech.edu/%7Eboyk/spectra/spectra.htm

The paper raises the possibility that ultrasonic frequencies may be perceived in a different manner through a different part of the inner ear. I have no technical knowledge in this area, so I leave this to others to debate.

Having heard two different speaker systems with treble extension significantly above 20kHz, I have personally experienced a greater sense of hall ambience and air from these speakers than I have ever heard through speakers that use more conventional tweeters. The dimensions of the recording venue - and, particularly, the height of the ceilings in the venue - were more apparent than I have otherwise ever heard from an audio system. It has been my running hypothesis that the additional treble extension produces this enhanced sense of space, but this is just my own subjective impression.

The Boyk paper makes no such speculation and it is, frankly, old news. As for the speculation (it was not a finding or otherwise demonstrated) in the referenced paper that the saccule might be a transducer for such high frequencies, it is hard to entertain such a speculation given the biophysics of the saccule. It is, rather, much better at extreme low frequencies, not high frequencies.

The Oohashi, et al., paper has been discussed in many forums and it is highly intriguing but far from convincing on several levels of experimental design and analysis.

Kal