I think Stevie is a comfortable one-octave zone, but I think her normal singing voice probably would be at its maximum at two and a half/three octaves. I know she can hit the higher notes, but she can't sing up the scale to hit it. She has to "jump" with her voice to get up-range, because I don't believe she can hit all the notes inbetween.
What I mean is, she has her normal singing voice (the one in the Stand Back video) you're talking about, and then she has this voice, the "high-note" voice.
She can sing this high, but she can't sing down the octaves to her normal singing voice. If that makes any sense. I could explain this a lot better if I had a broader knowledge of Musical Theory.
