Yea if it's supercooled with nitrogen or something lol a few degrees c aint gonna make you use less power.
So the water cooling uses no power then? I really doubt you get lower power draw just from water cooling especially taking into account the extra wattage from water pump and extra fans not in a measurable way.
Hey dude stop talking out of your ass. It's a known fact that cooler dies draw less power because of thermal resistivity. I personally watched a 55w power drop in a GTX480 as it was cooled from 100C down to 65C.
To answer the OP directly, you don't get a higher hash rate if you run your cards cooler, as long as temps aren't high enough where your cards will throttle. You do however save on power consumption, or give you a little more headroom for overclocking and stability.