i had S. repens in my low light dirted tank last year. Initially it was a low light, then I modified it to a low-med light with some extra LEDs. They didn't die under the low light... but I would say they survived more than thrived. They only did marginally better under the low-med light. Once I replaced the light for med-high they did better, but each lighting upgrade didn't help all that much either. It wasn't until I started adding CO2 gas instead of excel that they did well in the dirted tank. They might have done better from the start if I had been using CO2 gas instead of Excel.
Someone else out there might be able to say with certainty if S. repens is known to have poor absorption with excel.