So, I tend to agree that people overstate the differences between Genelec and Kef dispersion. However, the Kef R3 and Revel F208 are ~2dB apart from 1-10khz, which is the region you usually notice dispersion differences as translating to spatial qualities I think.
The real question is: How audible are changes in DI? I don't think there is actually a good explanation of that in Toole's book, even. My guess would be significantly more audible than they seem intuitively.
There is a study referenced by Toole about audibility of different directivity patterns. The graphs aren't exactly... the nicest quality... but their "direct radiator" design and "cardioid" design(pg 293) seem to correspond more or less to the differences I see between the Revel F208 and the Kef R3. And the answer to "was this audible" seems to have been yes. In their first experiment, they varied only the surrounds in a 5-channel system, yet there were significant differences even then. Question 1 is envelopment, Question 2 is detail.
In the 2nd experiment, they varied the front 3 speakers, and while more marginal than the 2nd experiment, differences were still audible(and proved that it's... extremely bad to have a significantly different center from your surrounds, lol). We know that surround systems make it harder to discern qualitative differences between speakers, so it's logical to predict that audibility of directivity differences is significantly more pronounced in stereo, let alone mono.
It's also important to remember that sound power DI uses the listening window, which ignores vertical dispersion past +/- 10 degrees. So, built into it is the standard "vertical directivity doesn't matter that much" assumption of the Spinorama, which IMO is one of its least well-supported assumptions.
I suspect that a better way to show differences in the overall sound field created by a dispersion pattern would be to calculate the critical distance for each speaker in a standard room. Genelec does this in their "correct monitors" chart and it leads to pretty big differences even though the dispersion of their speakers doesn't vary that much. The 8341A and the 1236A are different by 2x despite the latter's horizontal dispersion being at most (+/-) 10 degrees less than the 8341A's. Vertically there is a much bigger difference, of course...
Interestingly, the Neumann KH80, Kef R3, and Genelec 8341A are more similar than not up until about 4khz at which point Genelec diverges with consistently wider dispersion. I suspect that this, too, is audible and probably what leads to the common perception that Genelecs are brighter.
Good points but one thing we don't discuss much is not just how wide directivity is but also how smooth, the Revel F208 is a good example of a wider directivity but it's also very ragged, which means the direct sound and reflections will be different, how audible that is I don't know either.
The other relevant study I'm sure you're aware of is the mono vs stereo tests where there was a large difference in spaciousness in mono but the differences were negligible in stereo, that is evidence to me that we should really be focusing on the measurements being neutral and smooth in the listening window and early reflections but not worrying as much about how wide the dispersion is.
There is quite a big difference in smoothness between the lower end Revels like the F208 and the higher end like F328Be, however. I think maybe the spatial difference would be clearer if you were comparing those(or the Salon 2s, which are even wider). And I don't think those impressions would necessarily generalize to the lower end Revels either.
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