The napkin-math is pretty simple. It takes 8.33 BTU to change 1 gallon of water 1 degree. So, 1000 gallons from 65-80F means you've got a bank of 125K BTU. Cooling 200 gal from 150 to 80 costs 117K BTU. Actual time to cool is variable, depending on heat exchanger efficiency, etc. Redirecting 1st couple hundred gallons will definitely help but how much? The math gets hairy. Harder question is, how long will it take for the tank to return to ambient?