Never done it with a PC water pump, but in the field, we just crank up the voltage. It really depends what pump you are trying to
oc. Most good ones now are using an magnetic field and only a shaft as the rotating part. So if you increase the field strength, theoretically it should spin faster. There are other things to consider as well, NPSH (net pump suction head) is another. Putting two pumps in line isn't a bad way to go, We do it all the time in our business, they are simply called 'booster pumps' and they can typically increase flow rates by 50% of given gpm of 'booster pump'.