I had a '93 Plymouth Laser that I had put Water Wetter in, to help control temps while driving (and pulling a 2K lb U-Haul trailer) from SoCal to PA in July '97. Worked great; it got hot a couple times on the climb to the Eisenhower Tunnel on I70, but never overheated.
But when the first really cold days of fall came along in PA that year, my wife complained that the heater was blowing cold air. So I took it out for a spin and found that the temp gauge wasn't even getting into the "normal" range markings, much less into it's normal spot. Couldn't figure it out, until I remembered the WW. So I drained and refilled the radiator with fresh coolant, and it warmed fine after that.
SOOO, YES - Red Line Water Wetter does make your cooling system more efficient.