Sorry bvukich, I may have been wrong.
Well, looking at that review, I am
The reviewer claimed to have it tested alongside Canon 24-70/2,8, Canon 24/2,8, Canon 50/2,5 and Canon 35-105/3,5-4,5.
Sadly I can't provide you with a link, since, well, it's a website in russian language. The reviewer (who seems to be credible, i.e. it's not his first forum post) claims that when pitched against those lenses it produces a darker image at f/4.0 than those other lenses produce at f/4.5.
There's a huge list of feedback there, and only 3-4 answers seem to claim that this f/4.8 is indeed an issue, but I just wanted to hear the CR community's point of view. Seems those 3-4 people got faulty lenses or w/e.
By the way, could anyone please provide me with a link to some website where people review lenses, bodies and etc.? I'm reluctant to trust Amazon user reviews since the majority of Am's users are not photo enthusiasts, even if they are trying to review L lenses and such. I'd be extremely grateful