I am not a hydraulics expert either, but the more water you can run through your panels, the better. Your limiting factors should be the pump's capacity and the pressure the panels can tolerate without leaking.
Remember: you want the panels to generate heat, not temperature. If the panels are in direct sun, and water is flowing through them, yet they are cool to the touch, then the heat energy they are absorbing is being transferred to the water at maximum or near-maximum efficiency.