It hasn't been double every year. :\
Beginning with the first Apple A# Chip (iPhones 1, 3G and 3GS all had Samsung ARM chips)
Geekbench 3 multicore scores:
iPhone 4 - 206
iPhone 4S - 406
iPhone 5 - 1276
iPhone 5S - 2378
I dunno....looks like AT LEAST double to me. That's the 4 year history of the Apple A# chip. Could we see an A8 that tops 5000? Based on the trend....
Keep in mind, I'm going by Geekbench scores and not the percentages listed in their keynote. The Geekbench scores work cross platform so one can compare an iPhone to a Macbook Air.
For comparison, here's the last 4 years of intro level MBAs:
Macbook Air (13" - Mid 2011) Core i5-2557M - 4282
Macbook Air (13" - Mid 2012) Core i5-3427U - 5142
Macbook Air (13" - Mid 2013) Core i5-4250U - 5022
Macbook Air (13" - Early 2014) Core i5-4260U - 5302
The i7, top of the line MBA (mid 2013) that I own gets a Geekbench 3 score of around 6182.
So, see its not that far away. The MBA line (which makes for a great all around laptop) isn't going for additional power these days. A desktop OS like OS X has no problem on an MBA and yet its score is only in the mid 4000s. And while it's a bit noisy (chip runs HOT), I can game on my i7 MBA.
We aren't that far away. I predict an A8 that's on par with the entry level MBAs and 2 GB of RAM for the next gen iPhones and iPads. Wouldn't surprise me either to see next year's entry level MBA with an A8 chip running it either considering my iPad can last longer than my MBA does on a smaller battery.
Evidence for increase to 2GB of RAM:
iPhone -
128 MB LPDDR (137 MHz)
iPhone 3G -
128 MB LPDDR (137 MHz)
iPhone 3GS -
256 MB LPDDR (200 MHz)
iPhone 4 -
512 MB LPDDR2 (200 MHz)
iPhone 4S -
512 MB LPDDR2 (200 MHz)
iPhone 5/5C -
1 GB LPDDR2
iPhone 5S -
1 GB LPDDR3