You should be able to add a 3D object as a child of a RigidBody2D. At worst case you can attach a script to your model that makes it follow the RigidBody2D. You will probably have to build custom colliders to get it to match your model.
In terms of performance I don't think you will end up gaining much, if anything. I'm only listing this as a solution because I can come up with a couple of cool concepts using a 2D game with 3D graphics, or vice versa. But you need to build it in as a designed mechanic, not a performance boost.