40 PSI is on the very low end of operating an irrigation system. Especially if you take into consideration the friction losses for a backflow device, pipe, valves, and any other component which reduces the water pressure. I am not sure I am understanding your math correctly. You are saying you are flowing 7.5 GPM at 0 PSI? that is not possible. In order to flow a GPM, you must have some type of water pressure.
Tony Posey
Ridge Run Landscapes