I agree, anything intruding into the aperture in front of the lens will cause vignetting (except with telephoto lenses).
I know that hexagonal iris apertures will cause diffraction spikes but I don't think it will affect overall sharpness much. If the diffraction spikes are unwanted then rounded many-blade aperture irises should be used. Unfortunately, great glass and a round aperture don't always come in the same lens…
Cheers
Steffen.