[update]

Power differences are significant enough to justify the effort.

running at 900/750@0.925v my card draws 142w
running at 900/150@0.925v my card draws 125w

running at 1100/950@1.055v my card draws 236w
running at 1100/150@1.055v my card draws 200w

Dropping the vmem voltage from 1.6 down to 1.5 nets about 2-4w when running at higher memory speeds (700+)
but doesnt do much at lower 150mhz speeds (less than 1w, well within the scope of inaccuracy of my power meter)

kind regards