If you want to maximise the efficiency of your cooled mining card, there'll be an equilibrium point where a 1W increase in power to your cooling solution (probably a fan) would result in a 1W reduction in the power draw of your card. At this point it's not worth cooling the card down further (unless you go for a more efficient cooling technology)
True but one can also run a card cooler by lowering clocks. Lower clock enough and you can also lower the voltage.
I can run a 5970 @ 40% fan and <60C but only at 535MHz and 0.7V
Right now my power costs even with "hot GPUs" (~70C) are only about 1/3rd of the revenue so increased efficiency is mostly academic however as the network becomes more efficient (7900 series cards, FPGAs, etc) things like lower temps, undervolting, underclocking can be used to increase the "effective economic lifespan". When my 12GH/s farm is no longer economical I can "convert it" to a 6GH/s farm which is economical and grind out maybe another years worth of revenue.