When I got this, at level 36, I already had a NQ one, so I decided to do a test, I sang etudes on myself with:
1. Voice
2. Siren Flute
3. Rose Harp
4. Rose Harp +1
I'm pretty sure I had capped singing and instrument skills.
Voice: Boosted stat by +3
Siren Flute: Boosted Stat by +4
Rose Harp: Boosted Stat by +5
Rose Harp +1: Boosted Stat by +6
Just thought that was pretty cool, :)