Apart from improved insulation I don't think there's been much in the way of improvements since the late 30s --- if you look at the hermetic systems, they haven't changed much in decades.

In fact, the newer "environmentally friendly" refrigerants are less efficient than CFCs... so it's sort of a tradeoff between risking more ozone depletion when they leak, or putting more CO2 into the atmosphere from the higher power consumption...