I have been on the front row of more than one concert and within the first five of many many more - At every one, I have heard the sound from her mouth - so I say yes she has.
On the SA tour however, during Blue D., I could swear there was a harmony track playing for her to harmonize her lead with - no one else can sound like that
It is to me VERY audible in the Letterman performance.
But, I think Stevie is 100% live as is LB.