> Their photosynthesis processes evolved in and are ideal for an environment with CO2 concentrations at least 5-10x the current levels

Wait a second. For the last few million years CO2 levels have been in the 180-300 ppm range (except for recently where they have gone above that).

What makes you think that plants that initially evolved hundreds of millions of years ago in very high CO2 environments did not continue evolving as CO2 levels changed, so that the plants we now have find 180-300 ppm to be ideal?