Will the addition of salt further cool down ice that is already at -30 °C?
If you add salt to icy water, it's the water that gets colder. Pure water under atmospheric pressures will be $0~\mathrm{^\circ C}$ - even when the ice melting in it is colder (for example if it's just out of the fridge). When you then add salt to that water, it lowers the freezing point of your water-salt solution, all the way down to $-21~\mathrm{^\circ C}$ once it contains $5.2\ \mathrm{mol}$ of NaCl per kg of water. That's the maximum amount of salt this solution can contain at that temperature. (Find the thermophysical data at NIST (page 30).

Now, if you add salt to liquid water (containing no ice), you will actually decrease its temperature slightly because of the energy required to solvate the salt in the water. This won't help you in a 'water cooling circuit' because you also lower the heat capacity.