I design mechanical stuff for a living and I can't imagine that designing cars in VR is more efficient than CAD. There isn't any barrier right now from idea to CAD. If an engineer (or a scientist) can imagine it, an engineer will be able to design and create it digitally with current technology. I would say the current limitation is the manufacturing tools rather than the designing tools.
Side note, I think the work process for designing the exterior of cars is to create a sculptures with clay, transfer to clay to CAD, and rinse and repeat. I wonder if VR will ever beat manual sculpting.