Manufacturing is exponentially influence by production modifiers (manufacturing capital, star bases, factories, race traits, etc) and then converting it to income which in itself is exponentially influence by financial modifiers (financial capital, star bases, improvements, etc) which will generates an excessive amount of wealth.
The description states "Pumping out consumer goods from our factories encourages economic activity - and therefore the growth of our treasury." which mean it should increase the generated wealth and not the generated income.
PlanetaryProjectDefs.xml:
For ProjectEconomicStimulus currently reads <EffectType>ColonyGrossIncome</EffectType> it should be <EffectType>ColonyNetIncome</EffectType>.