It really depends on your other factors. The popular theory in biology is that the resource (eg. CO2, Light, Nitrogen, Potassium, etc.) in the lowest supply will limit your growth. So if you have all the CO2 in the world but are only running 40W of light or have a nitrogen deficiency, your plants will only grow as much as they can based on the lighting or nitrogen supply. So in layman's terms, unless you are dosing a full regime of ferts and have at least 60W of fluorescent light or T5HO lighting, CO2 won't make much of a difference.
For the substrate, plants go best in a special nutrient enriched substrate like Eco-Complete, Aquasoil or Fluorite. I have a mixture of 75% eco-complete and 25% black sand in the tank in my signature and the plants are growing great (even with no carbon and aboout 1 WPG of light).