As I have promised - here 3 shots which do perfectly explain what happens when you have barrel vignetting on the HCam, do a full shift and do a stitch anway.
It doesn´t matter - IF - the image circle of the actual lens is larger than the flatstitched area.
So you can use a full shift and gain much more image as normally expected. To explain the term a bit better, what actually vignettes here is the mount of the lens.
The samples where shot with the 4/40mm IF Hartblei (which is actually the Hasselblad 4/40 IF Distagon of the last V series - Image circle about 90mm)
Greetings from Germany