Just my 2¢:

Remember that the temperature of the water in the heater isn't what's going to heat your pool, it's the throughput of water warmer than the pool that does it. The more water you can get through your heater, the faster your pool will warm up, even if the heater water is only 1 degree warmer. BTUs/KCalories are what you need, not temperature. The limiting factor of how much water you push through your solar system should be the maximum gpm system can stand without leaking or the pump's maximum GPM, whichever is lower.