I was wondering if anyone knew exactly what this edge does. It's expensive and does not seem to improve song effects (strength) though it does seem like it might increase duration although I didn't record before and after duration. But the actual effects are no different before/after taking the edge.

Your instrument sometimes counts as a slightly better instrument for the same repertoire, and I believe it is more likely to boost poor or mediocre instruments than really good ones for the repertoire.

Which, depending on the song, can have a small effect or a big one. It's expensive but if I were playing an instrument-centric bard I have/would pick it at some point.

I.e. you can sing in epic repertoire with a harp and count the instrument's power as if you sang in romantic or close to it.