While I think it depends on your hearing abilities... I am starting to believe there is some merrit to extension beyond 20 kHz. We have the Kef reference in the store that I work at, which uses the hyper tweeter with extension up to 50 kHz or so. I do notice a certain air in their sound which is less prominant in speakers which don't have this extension. I notice this with similar speakers I build using tweeters with extension beyond the 20k mark.

Adam