The key thing to remember is that Tesla's battery competence has its foundation in their work with Matsushita/Panasonic - who are supplying the tech for the battery farm in Arizona. The main reason is that manufacturing and QA of the battery is critical. It will be interesting to see if Apple will work with Panasonic in this space.

That said, Apple of course may be able to sell at higher margin and be able to endure higher manufacturing cost.