The gap may also be due to shoulder fwd/back and twist. Are you using the arms down pose dial on the body, or moving them manually?

There's a lot you can do with joints by moving not just the primary actor (in this case the shoulder) but also the supporting actors like the collars.. or in the case of the neck/head by distributing twists and bends between them instead of cranking the head around 90 degrees, for example.

With 3D figures there's always gotta be a compromise somewhere: I would have happily made 100 JCMs, but then every single clothing creator would have to match each of those JCMs in clothing. It's a fine line :(