In Theory: IF whatever coolant mixture you're using has excessive surface tension (as straight water does), then Water Wetter should improve heat transfer between the engine and coolant, and between the coolant and radiator. As Shannow points out, this wouldn't really reduce average coolant temps in a car with a properly functioning cooling system and thermostat. What it could do is reduce the number and severity of "hot spots" in the engine.
In Practice: Don't Red Line's own advertising materials indicate that Water Wetter makes a minimal difference when using a 50/50 coolant mixture? I have a hard time imagining that any of the potential benefits would be noticeable at all, but YMMV I guess...
I tried it in my car. I stopped using it because it made brown bubbles appear at the top of the coolant expansion tank, which I don't want to see unless the head gasket has failed. I have no way to tell what effect it had on the efficiency of the cooling system.