co2 being used should meet a few things first, (light saturation) normally you'd use co2 with light at or over 75w per sq ft, 2nd temps need be higher with co2 to allow it to even work, adding a co2 to a room even if ya don't meet them things will give ya a slight increase, but I wouldn't expect even 20% increase, maybe 5-10% at best.. there are other factors to consider also, like how often ya plan run extraction etc.. coz if always on your wasting your money with co2 you'll just be sucking it out... I hope that helps a bit..
