Higher flow rates do equate to higher thermal transfer efficiency but only up to a point. Most manufactures recommend 0.1 GPM/sq-ft of panel to obtain maximum efficiency and forcing more water than that through the panels won't get much more heat into the pool but it will lower the electrical efficiency of the pump. For most panel setups, optimum efficiency is achieved at a point that is 3-5 PSI above the operating point without the solar panels.