I am starting to get 40 hours standby and 12 hours usage since I updated my iPhone 6 to iOS 9. Mow I can go almost two days in a single charge. My usage habits are the same as pre iOS 9. Thats without turning on Low Power Mode.
It is not the same as the A8 on the iPhone since the RAM is 2GB instead of 1GB. The iPad mini CPU/GPU pushes half the number of pixels and the device is much smaller. It is more important to control heat and battery than get small performance increase.
That's what I was told yesterday when I chatted with Apple representative about it. He told me iPhone Upgrade Program only available in store and what I need to do when preorder start is setup in store pickup to use that program.