Lighting an Aquarium with PAR instead of Watts
This thread describes the issue fairly well. ut if the Ray II has the same number of bulbs as the Planted+ then you are taking away a few bubs which do fairly well on
plants(as far as the spectrum being good, but not in any "sweet spot" for the plants)
and replacing them/w better bulbs(that are exactly in a sweet spot that the other Ray II doesn't cover.) I would use the same line as for the Ray II to see how much PAR you get from it and it says 45 at 20".
If you want to stay low tech, then it's good for that at 20". The problem/w the overall picture might be that most will want tall plants to try and "fill" that height and then any plants in the shadows of the taller ones lack good light. Anubias, Jave Ferns don't need much light so they could be planted in those shadows and do just fine.
If you haven't gotten the tank yet they do make a 40L which has the same footprint but 16" tall. The visible light from the Planted+ is less which leeds some to think it
might not be enough for a 55 but for one that's low tech it is just right IMO.