The purpose of the hole is to allow air and water to pass through the thermostat even when it's closed/cold. Especially air. It makes the burping process quick and easy. I have not bought a thermostat in a while. But I'v heard some of them already have the hole in them.
It will not changing the cooling efficiency of the system at all. You will not run any cooler.
Anti-freeze lowers the freezing point, raises the boiling point under pressure, lubes the water pump, and helps prevent corrosion. But pure water is a better coolant than an
antifreeze mix. It transfers heat better. With a surfactent, it will transfer heat even better.
I use about 25% anti-freeze, 75% tap water (I have very good water), and a bottle of surfactent. I don't have any problems running hot. The fan only comes on if it's a hot day and I'm sitting in traffic. On a cool day, I have to block off part of the radiator to get the engine warm enough to operate well.