I shot weddings with a Mamiya 7 for years, and much of that was handheld using available light. I never had a problem with the look or feel of the images wide open or camera shake at slow shutter speeds. Attached are some rather poor scans but they should give you a sense of the 80mm wide open. Can't recall what the films or exposures were, though. Likely in the 1/8th to 1/15th range I would think.
EDIT: In case you can't tell from the image, the hairs on the bride's forearms are tack sharp in the negative.
Wow! Weddings with a mamiya 7 - I am super impressed!! Makes me love mine even more!