I have the 200mm 4.0L IS version.
I just used this combo for the first time on a shoot. Didn't notice anything in particular and was at the 200mm range often. I didn't do any clinical tests though.
Do you mean some slight darkening in the edges, or actually being able to see a round dark shape from the lens. The latter I certainly have not.
I think the combo makes for beautiful shots. I feel I don't need the extra weight and cost of the 2.8 version. Love the lens for full frame photography too.
