You shouldn't look at CO2 as an instant yield-improving device, because it is more complicated than that. You can't just throw it in and "double yields". I also have never heard anything about only needing it if temps are high. A plant will withstand higher temps in a CO2-rich environment, but any plant will benefit from them. Really, for CO2 to be a big benefit to you, you need to have everything else perfect-- They need to be getting fed the right amounts, and be getting more than enough light. If they do not have more light than they already need, they will eat and breathe at a normal rate. So, before you worry about CO2, do you have everything else sorted out?
S-Town