If your cooling system is rusty enough, cleaning it makes a big difference. We have to de-rust my wife's Corvette every few years, or it starts to overheat. In a clean system, water wetter makes a noticeable difference. My big block Chevelle could just barely keep the temp down. With water wetter, I could idle in traffic. Water wetter reduces surface tension, which allows more water-to-metal contact area, and therefore more heat transfer.

So if you clean a really dirty system, and "wet" the water, you could conceivably gain 25 degrees. But I bet most of us maintain our cars well enough to prevent that much rust build-up.