I disagree with your assessment that Tesla doesn't have any new technology that will improve their efficiency over Nissan.

Tesla builds their own powertrains and battery management systems, which are good enough that other automakers license or purchase these components from them.

I also believe that Tesla has efficiency advantages due to the high degree of automation they've built into their factories, as well as the lack of dealership markups. Tesla's profit margin on the Model S is excellent compared to many other automakers, which leads me to believe that they will also be able to offer more for less with the Model 3.