it's not just adding co2 to what? 1200-1300ppm? You also have to control temps and RH% in order to take advantage of the extra photons, too. If you upgrade to CO2 or plan to, also upgrade your environmental controls.. .could get 25-33% more yield? Theoretically you can apply an average of 1500ppfd accross canopy at peak.. but don't expect to get there. (back to eh temp and RH%.. ties in here.. it's at the leaf where things happen, so how difficult wold that be to maintain environment around each and every leaf perfectly to maximize photosynthesis? better to aim below 1500ppfd even when running a tight ship.)