There should be a greater temp difference between the ambient ( outside air ) and the discharge air temp out the top of the heater than before. Where and how are you measuring the water temps and what are you using to measure? What is the rated capacity of the heat pump? At what water level of the pool is the sump pump ( stratification? ) and what is the rated flow rate for the pipe size that you are using? Would also need to know all heater operating parameters Eg: system refrigerant, ambient (outside air temp), discharge air temp, suction pressure, liquid line pressure, suction line temp (to calculate superheat), liquid line temp (to calculate subcooling), compressor "RLA" listed amp draw, actual compressor amp draw when running. With that information I might be able to tell what is going on with the system. Any idea on how old the heater is?