Expanding on what I had said above, it can't be more efficient. Heating 20,000 gallons of water will take x number BTU's no matter if you heat it from the bottom or middle to side or top. Heat is heat.

Actually your heater itself will be more efficient as the temperature of the incoming water is lower. So let’s just say you do have a thermocline in your pool and it is 85F at the top and 80F at the bottom. If you heat that 80F water it will extract more heat as it passes through the heater then it would if it were entering at 85F. Of course if you increase the flow though the heater as well you would extract more heat.

Again to heat a 84F pool to 85F it won’t matter where the heat enters, from the sun, a bit from the air or from a heater. As long as it is properly circulated it will take the same amount of heat to heat it up.