Why not, there will be for sure students who have the ability to grasp a course fast and effectily to compete with those with prior knowledge, why not grade and reward that ability.
The closest thing I ever saw to this was my high school weightlifting class. Your bench press single rep max was a high percentage of your overall grade, and was tested several times over the course of the semester. You were graded on improvement. The teacher made sure not to mention this before the first test, to keep people from cheating, but a few kids were in the know and faked a low max.
From a philosophical/values point of view- what is more important for education- incentivize all students to try to achieve the mastery level expected, or incentivize all students that as long as it looks like they're making progress, that is enough? I'd go with mastery. The appearance of progress is too easy to game, too subjective, and not easily applicable to varying courseloads.
Why should it be graded? The point of the class is not to learn quickly and effortlessly, but to learn the material completely. If by the time the class is over the student has learned it well, they should get a grade that reflects that learning.
I mean, by your logic, should we give extra credit to the students who never show up to office hours in college, since clearly they're learning it 'faster and (more) effectily(sic)'?