Thank you very much! That means that the performance quirks exist (at least) on the following cards:
GTX 460m (laptop)
The only difference between the indexed renderer and the array renderer is that one uses 6 indices to form 2 GL_TRIANGLES and the other effectively does the same thing internally using GL_QUADS but without an index buffer. This isn't a problem when just rendering quads, but everything I'm rendering in my engine uses indexed triangles.
EDIT: I've posted this on the Nvidia developer forums: https://devtalk.nvidia.com/default/topic/548150/opengl/performance-bug-in-gldrawelementsinstanced/