I don't understand this relaxed attitude towards using high ISO. High ISO is not an equivalent substitute for exposure; it should be used only if the exposure can not be increased sufficiently.
The DR of the 5D2 at ISO 1600 is already two stops lower than at ISO 100, and going to ISO 3200 decreases it by almost one stop. In other words, the real gain between 1600 and 3200 is tiny, between of 0.1-0.15 EV, the loss of DR is accordingly 0.85-0.9 EV.