Ok, so here is the final observation from this test, the optimal port extension for 24-70mm II F-4 L IS and also the +4 diopter, both on Superdome. Note differences in corner performance and field of view:
24-70II_PVL45_superdome_16.JPG 401.87KB 32 downloads
24-70II_PVL40_superdome_+4_10.JPG 418.19KB 26 downloads
If you need to work wide with this lens try it without a diopter, but still try to get F-8 or smaller for a working aperture to achieve reasonable corner performance. For reef fish, accepting that you'll be tighter on the subjects, the +4 diopter is quite nice.