To be fair to Canon, they have improved performance at very high ISOs to a significant degree. At ISO 3200 and above the 5D3 is the equal of the D800 in terms of SNR and DR. At ISO 25,600 both SNR and DR in the 5D3 is a whole stop better than the 5D2. That's a degree which would be clearly noticeable. At ISO 25,600 the 5D3 even has slightly higher DR than the D800, but only by 1/4th of a stop which would hardly be noticeable. However, these results are at equal print size. At the pixel level the DR differences between the 5D3 and the D800 increases to a good half a stop at ISO 25,600, although I'm not sure how relevant that is in practice, but it might be useful to know. If one were to crop a D800 image to 22mp, keeping the aspect ratio the same, perhaps as a result of using a prime lens which didn't allow one to fill the frame with the desired composition, then one might like to know that the DR would be 1/2 a stop worse than the 5D3 shot of the same scene using a zoom lens to fill the frame, if one were using ISO 25,600.
Nevertheless, the 2.5 stop DR advantage of the D800 at base ISO is clearly huge, and of much greater significance than the 1 stop DR improvement of the 5D3 compared with the 5D2 at ISO 25,600.