One of the things noticed using the 5D2 during the Great Zacuto Shootouts is that the in between ISO settings render cleaner images. I don't remember the particulars, but and of the multiples on the 5D2, 160, 320, 640, 1280, perhaps higher, gave cleaner vid than the normal ISO progression.
I don't recall if this was for all or just the 5D2. Worth checking their website for the comparison and then testing on your own.