Each tank is a tank.. each experience is a experience.. mother nature is not an exactly formula.
I notice that in my tank the area with less flow is the one under the outflow and plants there doesn´t grow so fast as all others..
So according to this i would problaby say that less flow equal to less growth ins´t that so?.
But than again we would have to measure all variables mentioned by "Parotet".
Concerning the CO2:
In my opinion for what i was able to observe in my tank along the time, high levels of Co2 does harm the plants.
Without changing any other parameter i only downgrade the Co2 and notice plants to be having a more colourful aspect and robust stalk cause they are not growing so fast.
There was a more sustainable growth. If you increase to much CO2 (macro nutrient) is just like if you do ignition on an engine and pump the gas without letting the engine to warm up and lubrificate all engine parts so that you could take an 100% profit from it. Get the Picture?
You can have the light you want or uptake Ei fert as much as you like and so on but it all leads to Co2 as this is the ignition for plants to assimilate the rest.
So here yes.. i assume CO2 as the major factor for a sustainable growth.
And finnaly.. doing the mental exercise in inverse we came to the the initial issue: The flow!
In order to have a very good Co2 dissolution in the tank, yes you´ll need good flow! Not faster.. Good flow is: efective/eficient/ and this doesn´t mean outflow velocity ou filter pump capacity liter /hour.
Another thing, you´ll probably also need to figure out (testing) what hardware positioning gives the best result. And for measure that is not a question of days but weeks.. be patient.
Assembling and being a day watchfull hobbiest to see improvements or not.
Notice: Seeing bubbles of CO2 going around the tank is not a good guide to assume a good efective flow or CO2 dissolution as so many people think it is. If you see to much bubbles maybe you should consider changing your Difuser also.
Regards.