CO² will increase your yield by 20% or more. CO² will not kill your plants they love it, it can kill you but the amount is very high. The method you are talking about using I do not believe can produce enough to harm you.

The problem with using your method is you can't regulate. Sometime it could be just right amount and other times the level could be above the point the plant can't process the extra amount fast enough so it just goes to waste.

CO² will cut your over all grow time as well by several weeks.