Leakage current is highly temperature dependent, and a good chunk of total power consumption for sub-45nm bulk CMOS processes.
Switching speed is inversely related to temperature -> you can reduce operating voltage to "compensate" the speed gain, thus reduce dynamic and static power some more.
So yes, improved cooling can lower power consumption by a noticeable amount; No clue how much it is at 28nm, but I wouldn't be surprised if it's > 10W for 70 vs. 40 °C on these.
I know what you said is true, my question was to see what kind of expectation from the OP.
Altho its possible to have 40c on all these cards with watercooling, but that means his ambient temp would be damn low. I highly doubt he can save much more than 10w.