I have neither lens (feel free to downvote me since I'm not answering the question you asked, I won't mind, promise. But it's not clear to me that you're asking the right questions).
The paragraph above the one you quoted implied that shooting both at f/8 will show little difference in IQ. This is true when comparing many (if not most) lenses. But the only time I shoot at f/8 is for waterfalls or scenic vistas; anything else within 100 yards I shoot wide open. Are you the same (do you care more about wide-open performance or stopped-down performance)? If so, is f/5.6 acceptable?
I know this isn't a lens-recommendation forum, but... what other lens(es) do you have? What focal lengths do you prefer (do you ever shoot between 18 and 24? Between 105 and 135?)? What apertures? What weather conditions? Why do you think you want IQ (large prints? pixel peeping? resized-for-the-web 1024x768 images?), and against what lens are you hoping to gain it? Have you thought about, in the same price range as the 24-105/4L, the EF-S 17-55/2.8 IS, which I've heard is very sharp? The EF 24-70/2.8L? An EF 17-40/4L plus an EF-S 55-250/4-5.6 IS?
In defense of the decision you're considering: last summer I thought long and hard about getting a 70-200/4, with or without IS, then chickened out and bought the 55-250/4-5.6 IS. It was a lot cheaper, and had IS, and wasn't big and heavy and white, and I don't make any money off my photographs, so it was a lot easier for me to justify the cheaper lens. A lot of the pictures I take with it aren't nearly as sharp as I'd like, but some are, and (in bright daylight) it's taken a couple of my favorite pictures! Still, though, I've looked at a lot of pictures that I've taken with it and wondered to myself, "what could have been if I'd gotten a 70-200...?"