For what it's worth, Notes of a Delightful sound in past editions granted a whopping +6 bonus to music rolls!
However, I always thought that was excessive, particularly when interacting with things like Enchanting music.
I think that an Imaginem spell should grant something like +2 bonus to Music rolls, possibly +3 or +4 if the caster is deft enough, for example if he makes a Finesse roll that beats the Music roll being enhanced. It's hard to improve the quality of master musician's performance, particularly if you are a lousy sound technician yourself! I do not think high magnitude should play a part in it.
I'd be inclined to grant +3 for a narrowish range of music rolls, such as to play a (particular?) instrument, or alternatively to sing. Think the sort of thing you can take a speciality in. It wouldn't affect all music rolls - in particular, I don't see composition being affected.
You're entirely right - I'd been thinking solely about MuIm rolls, and hadn't thought to look at other approaches. It makes sense that InIm could help composition (although via listening to the music of the spheres wouldn't have been my first thought!)