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.What Co2 really does is replace the ability of the plant to effectively "grow" at normal growth rates that the light saturation point removes during the normal grow day. This light saturation point hits the plant earlier then many think. It's (the plant) cells in the leaves actually begin to change on the peptide level. This change is to actually protect the plant from the intense light of day. This includes UVB protection.
This reduces active plant growth by around 30 - 33%+ per day.
At night, the plants reverses gears, and the cells change back to being able to use all the light energy they get.
Gassing will stimulate the plant to increase it's actions to negate the need to protect it's self. It then grows at normal rates, longer and you reclaim that 30% or so lost growth from light saturation points.
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.