Leaving aside the fact that the A7 actually is much fasteryou have to look at the bigger picture, as Apple often does.
This guy mentions ram as a reason for 64-bit. While that is true, what would you have Apple do? They're fast approaching 1 million apps on the app store. By providing a 64-bit chip today, developers will have plenty of time to ready their apps for a couple years down the line when mobile devices begin breaking the 4GB threshold. Apple will have a rock-solid foundation of 64-bit apps and years of hardware ready to go and Android will have nothing because manufacturers waited until there was "real benefit" to put such a chip in their machine. This should also enable Apple to drop 32-bit support from iOS before Android, which I presume would provide some benefit? By 2016 or 2017 the iPhone 5 and below probably won't get iOS 10...err I mean iOS X.
The other thing is the iPad. It will probably get more ram before the iPhone line, so it will need 64-bit sooner rather than later. And with rumors about a larger, "iPad Pro" type machine, we could end up with 4GB earlier than we expect!
Lastly is the prospect of A series chips in future Macs. Considering how much code OS X and iOS share, and considering Apple's penchant for low-power consumption and high performance, is it really that crazy? Apple successfully navigated the switch from Power PC to Intel, and while it's kind of a pain, I think it's within the realm of possibility that they could do the same with ARM. Consider this: the iPhone 5S benchmarks around 2550 on Geekbench, and the iPad was usually 10-22% faster over the past two generations than the iPhone. So it's possible the new iPad could bench close to 3000! Now consider the top-end MacBook Pro Retina benches around 13000, and it would only take four A7X chips to get within that range, right? Up their clock speeds and you could surpass that.
I know the numbers probably don't transfer as literally as I've laid it out, but it's probably a fairly close guess? Any CPU engineers care to chime in?
I'd love to get a quad-core A7X iPad Pro with 4GB of ram, 12" screen and some real pro-level graphics editing and web design apps. Perhaps it would bench close to a MacBook Air? Haha, someday I hope that I can remove the desk and iMac from my office and just have a bean-bag chair and a big-ass iPad. Make it happen, Apple!