Right now I'm sure you are correct, I have a 6+ and it's performance is fine, but things change. OS and application demands are only going to increase, the new architecture may well provide higher power efficiency and thus better battery life. Finally we don't know what other hardware tricks the processor and it's supporting chipset implement. In the last few processor cycles Apple has been adding hardware functionality to support specific device features such as security, image processing audio signal processing and motion/position tracking so it's not all just about the raw performance.