As long as you are not using more than 12 GPM, the 1/2 HP pump will add 40 PSI to the city pressure. If the city pressure is 20, you will be getting 60 PSI.
You'd be better off trying to rework the sprinklers, like trying smaller nozzles in them, which can make them spray further. A better match as a booster would be a 1/3 horsepower jet pump, but they can be hard to find.