The max growth rates are determined by how fast the enzymes can fix carbon. So........the Rubisco can only fix a certain amount of CO2. The other enzymes that fix the intermediate will be slowed to nil as they are downstream of the Rubisco and are being fed from Rubsico(these are assumed to be fully staurated since you said the CO2 is maxed). Rubisco tends to have the largest pool since it's the first step and has dual affinity for Oxygen as well.
So it is feeding the fixed Carbon into the next stage downstream. So all these enzymes are "full" as a rule. The limiting step is the supply of CO2.
So adding something downstream to an already full system will not drive it any faster. Excel does either one of two things: degrades into CO2 inside the cell, or is potentially incorporated into Amino acids, sugar etc.
"The route through which Flourish Excel™ is used by plants involves two main processes: a) adsorption and b) transformation. Because the active component of Flourish Excel™ ( See Fig. 2, polycycloglutaracetal) is charge neutral and of relatively low molecular weight it is readily adsorbed directly across the cellular membranes of most plants. Once present within the cell there are two possible modes of action. It may be biologically converted into CO2 and then utilized in that fashion. Or, it may be converted into any number of more complex organic compounds needed for the life processes of the plant (e.g. sugars, starch, amino acids, etc). These conversions (in either mode of action) are mediated by any of a variety of enzymes present (oxygenases, carboxylases, phosphorylases, etc). In order to determine the precise mechanism (i.e. down-conversion to CO2, or up-conversion to longer chains) further studies involving radioactive C14 tracers would be necessary."
this would suggest that adding both CO2 and Excel may help. But this has not been shown nor proven as far as I know.
I have added both, many have, but seen no plant growth improvement, but many use it for algae removal.
I've not noted enhanced plant growth with its use + CO2 gas(when CO2 gas is high).
This leads me to believe the incorporation directly into sugars plays a very minor role.
That is mainly breaks down and it is used as CO2.
CO2 is much easier and faster for uptake in plants than Excel and does not break down(but can off gas).