Would it not only reduce the growth rate occurring after the light saturation point (unknown). You would still get a normal growth rate until that point, and then a reduction in growth after that point which would still total more growth. At least until the point where damage to the leaves is occurring.
Got me thinking and doing some digging on the light saturation and came across this. While all species are different this paper is about as close of research as i can find generically speaking about cannabis and doesn't answer my question but it provides some interesting data. A lot of this stuff i am aware but was surprised at the Co2.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550641/pdf/12298_2008_Article_27.pdf
Without Co2 supplementation at 30C there are increased rates of photosynthesis up to 1500ppfd although clearly the benefits past 1000ppfd are reduced and this study does not show the reduction once the saturation point has been reached. I could not for the life of me find a study that would come close to giving me a light saturation point in cannabis. Probably because one does not exist yet. It also shows that Co2 has a larger effect than i previously thought even at lower levels in those conditions, but since it stops at 750ppm you can't tell how much it increases beyond this point since it would again be on a curve. And because of the duration of light this study doesn't provide a good overall picture of how many ppm Co2 above 750ppm in relation to differing ppfd will increase photosynthesis or not over a full 18 or 12 hr cycle.