The power savings is substantial, but it's certainly not 40 watts per card (unless you were running the cards very hot on air in the first place). My water cooled 7970 machine saves about 27 watts (DC) per card (at 36-37c). Keep in mind that some of the apparent power savings is simply from not having fans on the cards though. Some of this power to remove the heat gets consumed by the pump and radiator fans.
They were running about 85c on air. So pretty warm. Went to about 40c on water. The fans do count as power savings in my case too. I already had the cpu watercooled and just add my gpu's to the same loop. No extra pumps, fans or anything.