im sure it produces some co2 and may be of some benefit to your plants but it probably isnt increasing the levels by that much so not sure I would even compare it to a tank setup how long a tank last really depends on room size and ventilation timing but 7 days isnt very long and you could cut cost by only using it during flowering not sure it would be worth using in veg.
jamstigator its my understanding co2 can double your yield in a properly set up room so it might be worth it for alot of people but obviously you are satisfied with your yield already would imagine just positioning your light correctly in your cab would probably help out some.