OP -
Wearing the corset as much as you need to for corset training would I imagine not be good for your spine, or ribs. I've heard of people cracking ribs before through wearing a corset, them continuing wearing it and the ribs not healing properly, causing rather serious scarring to the muscles. Furthermore, as a dancer I know that flexibility is absloutly crucial to you, and I can't imagine having to spend all that time with your spine perfectly curved and possibly compressed, without being able to move any of your vertebrae can be good for that.
Just keep that in mind. Also, I really think going below a 22" waist is just getting a bit extreme to the point of unattractiveness. As numerous people have said so far, you're stunning the way you are at the moment, truely. You don't need to do this.
May I add, from wikipedia:
The volume of the lungs diminishes and the tightlacer tends to breathe intercostally – that is, with the upper portion of the lungs only, rather than the whole (see shallow breathing). Intercostal breathing is what gives the image of "heaving bosoms".[citation needed] Due to the lower portion of the lungs being used less there is often a mucosal build-up there; a slight and persistent cough is the sign of the body trying to clear this (and might also have led to the Victorian hypothesis that corsets caused tuberculosis).
Dancing is pretty strenous, and I believe the ability to breathe is generally pretty advantageous no?