You can fit them at any power but due to fuel delivery being more critical the higher you go power wise, they become a must have rather than a nice to have at over 500hp I would say. Without them you need to add extra fuel to compensate for the likelyhood that 1 cylinder is running weaker than the others.
Remember when the AFR's are taken they are an average of all 4 cylinders, parallel fuel rails can get you closer to having equel fuel across all 4 allowing the fueling to be leaned out, this gives more power.