if you upgrade the pups to any single coils you have to use the proper dummy coil to avoid humming or bad sound symptoms. that is 'cause the dummy has to have the same windings as the three pups to work properly. the other solution would be to put noiseless pups (SDuncan or DiMarzio blades and single-sized 'buckers, Lace Sensors, Kinman, Fender Noiseless, Bill Lawrence etc.) and take off the dummy coil.
the original pickups in the Powerhouse are the same as of the American series strat+the dummy coil. not bad but not great too. the mid boost is a nice future when you learn to use it. just keep on mind it's always active and have effect in the sound and you always need to have a battery in the circuit.
IMHO, if you find a Powerhouse strat with no quality control issues it's a great buy (i'd say the same for the Deluxe Players, Jimmie Vaughan, Robert Cray and the Fat Deluxe MIM).