I know you can get up to 30% additional growth using CO2. But, the only way I can think of to use CO2 and keep the heat down is to use an air conditioner. Unfortunately, AC units use copious amounts of electricity.

So I'm thinking for people trying to keep their power consumption under control, might it be better to run another light and a vent fan instead of CO2 with AC?

I'm just thinking that one could do a lot better on a per watt basis this way.

You can have temps in the 90's using CO2, so you may not need an AC unit, escpecially during the winter.

Are you using cool tubes or something similar? You can eliminate much of your heat by putting your lights on their own intake/exhaust system. If you seal it well you shouldn't need to worry about smell for this exhaust, just push the air outside.