...this may be inspiring.
I have a Rolleiflex 3.5B and I changed the original screen to one designed for the Mamiya RZ67, going by these instructions: http://www.panum.de/rolleiflex_screen.htm
(for focus adjustment, read the comments further down on that site for doing it properly).
Comparisons have been done before, but I took a couple of shots before and after for my own sake. Since sharing doesn't hurt, here's the difference...
Shot in the same lighting at the same exposure. What looks a bit like vignetting on the new screen is just the shadow from the lampshade above the view. The vignetting in the left shot... is just that
Well, that cured me a little bit from craving a GF670W... at least for a while. It'll be fun to see what it looks like in daylight tomorrow!
Have a nice evening!