Correct me if I am wrong, Catchlights. According to what I understand from film users, the type of aperture used will affect the coverage of the flash. Therefore at F2.8, the coverage of the flash using E-TTL will be minimal and hence there will be slight vignette at the wide end which may not be noticeable on 4R but obvious to photographer's eye at 8R. This observation is applicable to big group shot.
I personally also adhere to the rule of minimal F5.6 with understanding. And I as a paid photographer will also never shoot at 17mm for group shots. Shooting at 17 mm will cause the faces of the pp at the side to be warped. If forced to shoot at 17mm, must ensure that i am at the correct elevation.
Adding to this fact,I will have to zoom in a bit to prevent distortion of faces at the side and this means that F2.8 becomes pretty unusable for group shots. It will be pretty obvious on 8R and above.