CleanCloths has gotten to the nub of the matter: An increase from 12kBTU per hour to 18kBTU per hour. While it's true that more flow generally means more heat, I suspect you had blocked off half your panel and therefore were only getting half the heat. Replumbing doubled the length and with it the resistence and GPM, but increased the heat absorbed.

That's my guess. Normally that "disk" or valve forces the water coming in one port to go the length of the panel one half (1' width) then come down the other. Opening the valve allows the water to flow through the end tube without hitting the panels.

Bottom line: You BTU/Hour added to the pool jumped 50%. And BTUs are REALLY what heating is all about, not temperature.