That's not how undervolting works though. You are just optimizing your CPU basically. So instead of using 1.3 Volts for example, you can achieve the same clock speeds / performance while running 1.2 Volts. That's why your system becomes unstable if you overdo it, because your CPU can't run at the same speeds on too low of a voltage. The values used in the BIOS are very conservative so every CPU can be stable. But most chips can achieve the same clocks with less voltage (that's why the newest blade comes undervolted from the factory), thus saving energy.
So if anything you actually gain performance because your CPU runs way cooler while also saving energy. Not much of a downside to it aside from modding the BIOS (which you'll have to do anyway) and some testing to find stable values that work for your particular chip.
