I've read that 5.1.4 is "better" than 7.1, but don't have any experience with it.
That being said, I wouldn't go by that image from Dolby. There are a lot of other "real world" tweaking and testing that people have done. Normally in a 5.1 setup, the surrounds are on the sides, but Dolby does have that right by putting them a little further back, although you could argue that their diagram is misleading as the speakers are closer to the 110* mark, if not a hair more but their line makes it look like they are in the middle between 90* and 110*.
So that is good still where they are at. I would nudge, ever so slightly the rear overheads in a touch and forward a smidge. In more than forward. Still keep them behind the listening area, and outside to the right and left, but closer to being more overhead (I said "closer to" not "make them overhead").
Next I would bump the front overheads (Speakers #6) closer to the fronts. This will allow for better DTS:X playback as well as help to "fill the gap" between the main front soundstage and the rest of the speakers.
Minor tweaks, but it is workable more so than a 7.1.2 setup.
