If ISO only going to 6400 means that results are better at 100, then so be it.
how is that?
can you explain the logic behind it?
from my (limited) experience the higher the max ISO goes the better the low ISO is.
Every step in ISO halves the sensitivity of the sensor, e.g.:http://sensorgen.info/CanonEOS-7D-Mark-II.html
... and the 5Ds will have a similar pixel size and thus similar full well capacity to that. For ISO levels above 6400, regardless of whether or not it is clean, there's very little left in the way of signal to work with (for detail work.)
With a shrinking pixel size, beyond a certain ISO level it kind of becomes pointless to even be shooting raw.
Beyond a certain point, high ISOs are just a marketing gimmick. Look at what you get from a 1DX if you shoot at maximum ISO.