As I am trying to prove in my 'high light plants in low light setup' thread. The amount of light needed is an overplayed part of the jigsaw. It is the driver of how fast and what form the plant will take but not a definitive will grow or won't grow.
You would probably need to go as low as 0.5WPG T8 to start worrying.
The more light you have a plant will grow faster and more compact, the colouration may also be dependent. The lower the light it will grow slower and more leggy. Red plants may be mre yellowy/peachy.
They will still grow though. Higlight conditions are not IMO needed for plants like HC and glosso but if you try them in low light they will not carpet, they will grow much taller.
Therefore your question of 1.5WPG even if it is T8 means that IMO you have sufficient to grow anything. No CO2 will also inhibit the speed of growth a little but it will still grow most plants.
Although I do use pressurised in my low light setup as I see it you are adding Easycarbo so in theory yours should have even more consistent levels of C supplied. There are many examples of high growth in non CO2 supplied low light tanks just as there are in the CO2 enriched. LLj on TFF has many journals of Non CO2 tanks and they all have superb growth.
I think people need to start looking at high light as a requirement only to gain a desired 'effect/result' from certain plants rather than wether they will or ton grow as this leads others to assume they have to get high light or they cannot grow plants.
I think the equilibrium Ray talks about is a simple case of providing everything the plants need for thelight given, therefore wether light is low or high making sure that nutrient and C is at the level the plants require for the speed they are driven. this can of course mean that low light may have enough C and nutrient from the content of the water from gaseous exhange / fish waste or food / organic waste and anything the substrate contains already. Not all tanks are the same though even if they have identical size, light, fish, filtration etc. Its a personal judgement call here which is why it easier even in low light setups to dose (lean is OK) and add a source of C (CO2 or Excel/Easycarbo) to rule out any defficiencies completely.
AC