That is a fairly poor way of exchanging the heat from the water and the mach unit i have to say, most people put the coil on the inside of a large ice chest, then you can get WAY below 2C, the thermal transfer is far more efficient and it will be able to handle load (i dont think that design could handle much as the heat transfer is so poor)
Check out XS forums for some amazing heat exchangers and great ideas on water chilling.
Sorry if this seems harsh but compared to what is around that is pretty amateur, with a bit or thinking and work you could improve the temps a hell of a lot.
G