Anna, that's essentially how I made my own gathered bodice pattern. Here's a picture
that shows the dancing dress bodice. (Warning - the picture is quite large.) I'm not sure of the bust/waist difference, but undoubtedly it's more than five inches.
I didn't swing the pattern out scientifically. (Meaning I didn't measure it.) But it was significant, and the side seams are at quite an angle. I was worried before I made my muslin that it would be too full. Boy, was I wrong! I was a little disappointed, actually; it's definitely gathered, but not really that full at all. If I ever get around to using a really delicate sheer, I'll definitely draft a fuller pattern.
I'd advise that whatever you go with will not be too much.