I got one.
In horizontal position, bottom right and bottom left corner gets slight vegnetting. All other sides are clean and sharp.
In vertical position, all along the top edge is vegnetted a little. That side isn't clean and sharp. All other sides are fine.
There is no way to produce anything other than 6x8.
If you use mask (never heard of it except for the mask you'd place in the focus screen area), you should be fine. I think they are stretching a bit on this 6x8 back. It's really not 8 cm wide either. It's actually 7.55cm in horizontal and 7.5cm in vertical.
EDIT: by the way, I do have an SD body.