At first glance, it seems like the the organ piano in the Dark Sanctuary can play 16 notes. The 8 normal ones, and 8 that you can access with the Shift Octave button. However, the regular neutral input (●) plays the same note as the shift up input (⇧), and as such they can be used interchangeably.
Should this be taken into account when listing the songs that the game recognizes? Sometimes, it's easier to play the shift up note. For example, His Theme would go from this:
⬃ 🡆 ● ⬃ ⬁ ⬁ ● ⬃ ● ⬃ ⬁ ⬁ ●
To this:
⬃ 🡆 ⇧ ⬃ ⬁ ⬁ ⇧ ⬃ ⇧ ⬃ ⬁ ⬁ ⇧
Almost eliminating the need to pay attention to octave shifts.
Even Piano that may not be played that well can be made arguably easier by changing all but one of those neutrals with a shift up:
⇧ ⬁ ⇧ ⬃ ⬊ 🡇 🡆 ⇧ ⇧ ⬁ ⇧ ⬋ 🡆 ⬊ 🡇 ⬋ ⬋ ⬋ ⬋ ⬋ ● ⬊ 🡆 ⇦ ⇧ ⇧ ⬁ ⬊ 🡇 ⬋ ⬋ ⬋ ⬋ ⬋ 🡄 ⬉ 🡅 ⬉ 🡄 ⬋ ⬊ ⇧ ⬁ ⇧ ⬃ ⬊ 🡇 🡆 ⇧ ⬂ ⇩ ⇨ ○