CO2 helps out photosynthesis, and I'm pretty that it doesn't speed the flowering time, it just make your buds grow bigger before they need to be harvested. Ethylene doesn't start degrading unless the CO2 goes above 2000 ppm, which it never should anyway. So, if I were you, I would keep the CO2 up.
What you should be more careful about is the nutes your putting in your plants. Check out what mel frank has to say about balanced nutes during flowering, you'll probably find that your npk isn't doing too well.