In CA all fixtures are 1.5 gal/min flow. You can drill out the pipe mounting side of the shower head for a little more flow but the mixing valve is not designed to flow much water either. Unless you have the old school single valve for hot and cold. Another place to check in the pressure regulator. It is on the outside cold supply usually next to the main gate valve. You can measure the pressure at an outside faucet, then try turning the nut to increase pressure. Most houses reduce the pressure to the inside as high pressure puts extra strain on the little electric solenoids to your ice maker, dish washer, etc. As noted, a booster pump will definitely achieve your goal. Sounds like a fun shower;)
