Thanks for posting the Zeiss data sheet. Using the published exit pupil data for the 35-70/3.4 I was able to check the SB design and can verify that the SB itself won't contribute any vignetting at all at the 35mm end. In other words, 100% of the vignetting seen in John's 35mm/3.4 test shot (25mm f/2.4 actual) is due to the attached zoom lens.
The exit pupil distance of the 35-70mm/3.4 grows from 57mm to 78.6mm as you zoom from 35mm to 70mm. For an exit pupil distance of 78.6mm its possible to get a trace of vignetting in the extreme corner with the Speed Booster, but most likely the zoom lens blocks the pertinent rays before they even get to the SB so that the SB effectively contributes no vignetting throughout the zoom range.