You'll get a little more but the valve body and shower head does most of the restrictions. The EPA requires not more than 2.5 gallons at 60 PSI. What type of pipe you have now? On a well or city water? An old shower valve body or newer one? Any water flow issues at other sinks? Remove the shower head and with a five gallon pail, time it for one minute and measure the amount of water in the pail.
A shower head nozzle's can get pretty clogged up with hard water deposits. Remove the head and soak it for a day in white vinegar. With an old tooth brush, run the bristles over the shower head and rinse well. May need another soak for a few hours if it looks bad.