Another option is to downsize the impeller but the savings in electricity won't be the same as downsizing the motor. Just made a measurement on my 1 HP Hayward Superpump (circa 1997) and under full load the motor pulled 7.4 amps. Running dry at no load it dropped to 6.1A which is no great saving. A downsized impeller would be somewhere in between. I can't easily measure the true power consumption but the ratio should be close.
Al