You can use a water discount to speed trace time and produce a harder bar quicker. A water discount produces a stronger lye solution, because of course, the ratio of lye to water is higher the less water you use. Some soapers prefer to work with the terms of "lye solution concentration" instead of "water discounting".
And yes, using a water discount means less water to evaporate during cure, resulting in soaps that are truer to the size and weight you cut them.
You don't usually want to use a water discount if your using any accelerants or doing intricate swirls because you will achieve trace much quicker and it can get to thick trace before you know it.
Water discounting doesn't affect superfatting.