All things being equal, there should be no difference in power, with or without. The venturi in the throat is much smaller than the inlet to the carb or the inlet to the velocity stack. If you put the stack on without changing the high speed mix, you will get a loss of power as the engine will be running richer because it's taking back in the mix that would otherwise have been blown out into the airstream. All the stack does is increase the mileage a bit and sucks back in the fuel that would have otherwise been blown around inside the cowl and making a mess. More so at idle speed.