That means very little. HDPE wall thickness is typically about 0.1 inch, so the pipe wall presents less thermal resistance than the first 1/2 inch of soil surrounding the bore. That alleged "efficiency" has little/nothing to do with the piping material. The fact that DX can (sometimes) get by with a shorter loop field is almost entirely due to the much larger delta-T available with DX. Copper pipe is used only because plastic pipe isn't suitable as a refrigerant container.