If the rumors of the A9 chip going to a 14 nm process are true, then you'll see a huge increase in power efficiency or huge bump up in performance (higher clock frequency) or a combination of the two. And that's without accounting for any micro-architectural changes that we will likely see.
The A8 is basically a die shrink version of the A7 with enhancements to the memory interface. The huge performance bump occurred in the efficiency with power usage getting cut in half, while the performance increased by a more modest 25%. Most of the power savings got applied to the larger screens and higher resolution graphics processing.
With the A9, Apple does not have to account for a screen size and resolution increase, so they can really push the envelope on power efficiency or chip performance. With the tri-core A8X, Apple has already shown what they can do when venturing beyond dual-core chips. Lots of possibilities for the iPhone "6s" and "6s Plus".