need to keep in mind the following:
with 2 static meshes - e.g. a box with a lid that is animated… the game takes 2 draw calls and each mesh has its own transform.
when you switch to a skeletal mesh, each bone in the skeleton has to be interpolated between keyframes, and sent to the gpu in a uniform buffer, and a significantly more complex shader is utilized to multiply the transform matrix for each bone and apply to each interpolated fragment.
It is such a vast oversimplification to say a skeletal mesh saves a draw call and imply it performs better.
btw - skincluster is a maya term - implies two separate skeletal meshes.
If you want your top to come off, just use two separate static meshes.