CO2 helps photosynthesis. It would probably keep increasing up to 100 percent assuming the light was on. The plant probably needs O2 at night. They generally don't make greenhouses more than about 1000 ppm because the improvement isn't worth the cost of pumping in all that extra CO2. The amount of light that the plant absorbs limits the benefit so it won't increase in proportion to the amount of CO2 past a certain point. It wouldn't be much improvement past about 5000 ppm from what I have heard before. There is a significant improvement like maybe 50 percent and sometimes more for some plants at about 1000 ppm. The amount of light a plant can take depends on the plant. Most plants will probably max out at about the intensity of natural sunlight.
If the CO2 is high then it can take more light and still increase. If the light is low then extra CO2 won't matter as much since the amount of light will be the limiting factor. The plant can't absorb much more than natural sunlight or it will have problems.