[voyager2]

Greenhouse operators often provide their plants with an atmosphere of 1200-1500 ppm CO2, which would fall into the economically advantageous range. No idea where the "too much" range starts though.

[H8crilA]

Which is still waaay more than in the atmosphere: https://www.co2.earth/

Once we get to 1500 ppm the situation will be so bad that worrying about photosynthesis quality will be as unimportant as worrying about who wins the football cup.