With a coolant at 25C, 5 degrees above the ambient temperature, I reached about -10C under max load on the CPU. This is where the variable powersupply comes in. These temps were without having tweaked anything at all, it was just "plug and play". After adjusting the voltage to the peltiers I ended up at -18C, 8 degrees lower than before.
The reason to this is that peltiers have an ideal curve: you can increase the voltage to a certain point and still get better cooling, but when you pass that sweet-spot, it will start producing more heat than chill, i.e. the efficiency decreases. What you want to do is to find the sweet-spot and stay there.
Fig. 20 The digital thermometer.
I'm sure you're all waiting to see some benchmarks, so just click your way to the next page.