Remember, a 3 kWp Solar Panel, whether Monocrystalline or Poly Crystalline have been tested for 3 kWp in the factory before shipping; and will generate the same power output and you are only paying for the Wp output of the solar panels, not for the technology!
Thus, in all practical conditions, the only difference between Monocrystalline solar panels and polycrystalline solar panels are that monocrystalline modules can save some area (maybe a few square meters) needed to generate the same output; There is no change in output.