You don't need CO2 for better yield. You would need more heat to open the stomata, increase metabolism and respiration, and the increase would be negligible. I'm sure he'll be fine by harvest, without spending extra on heat and CO2.
Stomatas open in good conditions not due to heat. Plants use their guard cells to close the stomata in high heat to conserve moister. So plants like C4 don't open them till night time when the heat is at its lowest and humidity is at its highest.
With the addition of CO2, heat can be higher since the stomata doesn't have to be as open to get enough CO2 to complete the dark reactions in the leaves.
While I will agree with high heat caused by CO2 generators can almost cancel to good of adding CO2, and tanks are expensive and add another degree of complications. If you use the CO2 generator as a source of heat then you can and do get a boost in growth. Providing that all other conditions are optimal.
However, CO2 really isn't too cost effective, and is not used unless the grow environments are sealed to prevent pest and desease. Replacing bulbs more often will get you more bang for your buck than adding CO2.
If you want to add CO2, make sure you have nothing else you can do. Perfect your nutrients levels, keep the enviroment with in the optimal range, if you growing in soil, then ditch the soil and go Hydro. Many better options that can be used before investing in CO2.