The only good point you've made is the elimination of one of the dimensions of movement. However, the combination of piece value and target file is much more significant than a 10% compression of the game. For example, at any given point in the game, there's very few different moves possible from those conditions. The non-moving pieces are non-moving; even in chess notation nobody cares about those.
Additionally, the goal of this exercise is to deterministically convert chess to music. So I think you're missing the point if you count that as a negative.
Calling this 20% of music is a bit harsh as well. True, many elements of music are not represented here, but arguably this is adding an additional element.
Oh, so there you are!