I'm running a CO2 burner in my hydro room. It's a closed environment, no outside air intake and no venting. I can do this because I'm running LEDs and T5s. If I were using HPS, I couldn't run a closed room. I'm using an Atlas 3 controller which allows me to set a maximum level and then automatically sets the low level 200 ppm lower. So if I'm running 1500 ppm, the burner kicks on when it hits 1300 ppm. It also has a photo cell so it only works during lights on. I have two 18" barn fans for internal circulation. I learned the hard way how important good circulation is. The two plants that were right under the burner suffered from C02 acidification. The pH on that side of the pots (15 gallon), dipped to 5.6 while the side away from the burner stayed at 6.4. I do see a difference, especially in veg as that article stated. It's also important to cut the levels way back in late flowering. If not, the result is loose buds. I'm not seeing the 30 or 40% increase that some claim, at least not yet. I'd peg it more along the lines of 15% or so.
The people who are using CO2 with HPS lights are using sophisticated environmental controllers that control their burner, exhaust fans, A/C, humidifier and dehumidifier. They basically turn everything off when they are venting, then back on after venting so they they minimize the amount of A/C and CO2 loss.