No the lumens isn't the most important factor. Lumens is a measure of visible light, it's a measurement based on human eye not how a plant uses light. PAR is a much better measure, photosynthetically active radiation. It takes into account which portions of the electromagnetic spectrum a plant uses to drive photosynthesis. You could put a plant under a billion lumens of green light and it won't grow at all because all the light is reflected off the plant.