Absolutely, good insulation is recommended to protect the coil from the outside temps. However, you continue to be fixated on the idea that you will loose or diminish your delta T in the horizontal trench if the supply and return pipes are not insulated. You have a lot of Water going through the entering water line, which usually drops down to 32F, and is mostly surrounded by soil warmer than 32F. Trust me, the impact of a 5F colder pipe next to it is minimal. The surrounding soil will win. Your loop design and pressure drop is much more significant for overall system efficiency. You must pay attention to that one. 1" pipes in boreholes raise a flag in general, so does 300ft one way for the header pipe, especially if it is only 1.25". The header pipe itself has over 30ft of pressure drop if you are flowing 15 gpm through it. Tell us about your exact configuration, tonnage, how many heat pumps, pumping strategy and antifreeze. A diagram would help.