Well, it's been awhile since I saw actual numbers on CO2's performance, but from my recollection, high levels of carbon dioxide is a little more than marginal in its effects. Mmmm! Monsanto! Yummy! Joking aside, there are many varieties of plants, and it isn't always easy to get fast growing, or higher yield, and retaining other qualities. If you look at the amount of heirloom tomatoes, for example, there are dozens and dozens of different types, with different colors, flavours, yields, etc. You can't always get everything in one plant, you know!