While I don't get all the plumbing (amazing job!) I can tell you that generally, the faster you move the water through the system the better. It's not about temperature, it's about BTUs /KCalories. 1000 gallons 1 deg warmer will add far more heat than a teapot full of boiling water. Usually, the limiting factor is the pressure the plumbing can stand without leaking. This mistake is frequently made with solar heating systems--people assume their panels have to get the water good and hot and then trickle in. That's wrong. When I split my solar heating system into two separate systems, nearly doubling my water flow, my pool heated up far faster, despite the fact that my surface area is unchanged. Why? Because the maximum flow through the panels could be effectively doubled. You've actually done that with your 3/4" plumbing in the heater.

Think of it in reverse: Would your car radiator be more efficient at cooling your engine if the coolant flowed slower or faster?

Heat energy, not temperature, is always the key in heating systems.