A good article relating to your question is well addressed on Tropica's web site, see CO2 and biology of aquatic plants(Claus, Ole and Troels).
Read their conclusion also.
hbosman is right on track.
Your plants are much better able to use the light much more efficently, since they will allocate more of the resources to gather light, not CO2.
So you have much higher light use efficiency with CO2 enrichment.
In lab studies, they get 10-20x more growth using the same light and CO2 enrichment.
Excel is not a bad in btw method either.
You have to look at plant growth holistically however.
It's not just about dosing ferts, or dosing CO2.........or light, it's all of them together.
If you add more CO2, then expect more growth, more nutrient demand,m higher light use efficacy.
More light= more CO2 demand= more nutrient demand
Less light= less CO2 demand= less nutrient demand.
and so on........
For nutrients, enriched sediments make remembering to add liquid ferts easier and provide a redundant back up. Likewise, adding liquid ferts allows the sediment to last longer and less transport of nutrients for the plants/less risk of deficiencies specific to the sediment alone.
So a mix of both works nicely.
If you have a decent fish load, then often they can provide a good % or at leats back up supply should you forget, or if you wanna dose NH4(better to have the fish do this than monkeying with urea or NH4Cl etc).
Read the article on Tropica's web site, I think it will answer things well for you.