Ok, sort of a late response to my own question, but perhaps this will help someone.

Turns out in Polaris based cards (470/480/560/570/580) GPU core voltage will always be higher or equal to memory voltage. So even if cvddc is set to 850 but if mvddc is set to 900 both will be 900.

This has to do with how the core/memory voltages are derived from the board implementation.