ARM may not give equal performance, even with the v8 architecture, but consider how many people use mobile devices as their primary 'personal' computing device. What I mean by 'personal' is web browsing, reference look-up on google, etc, music, videos/movies/television (youtube, itunes, googleplay, etc), light gaming...
That's all most people do, as in the vast majority of people, most of the time, as in the vast majority of the time, outside of work related computing tasks.
So, given that a 2-4 core Cortex A9 is roughly equivalent to a 1-2 core Pentium III of relatively high clocks, and this is what most people are happy with for personal use devices...
Let's extrapolate and speculate, without any data of course, and say that a 2-4 core ARMv8 cpu is somewhere in the performance envelope of a 1-2 core Pentium IV (maybe slightly more than that?)... hey, more processing power for a mobile device at the same power requirements... great for normal 'personal' use, more so than an ARMv7 cpu.
Putting ARM aside, it doesn't matter whether it is ARM or Intel Atom, small personal computing devices are more and more becoming the norm rather than the exception or the add-on purchase. And it will only continue as mobile cpu performance and mobile computing devices increase their product specs over time.
It's not that much of a risk for Apple to introduce an ARM based system as a bridge between their tablet/phone ARM devices and their x86 systems. Something like a dock to plug into one of their large displays that allows the docking of a future iPad or iPhone, plug in a keyboard, go to desktop mode... no brainer to think that one out... they're already implementing some convergence between iOS and the current version of OS/X.
A 2014+ era tablet/phone, quad core ARMv8 (or Atom for a Windows version), 4GB RAM, gpu cores capable of pushing enough pixels to handle a 4K display resolution --- all that is a reasonable expectation. The question is whether it is profitable, and that all depends on how a company like Apple structures its hardware offerings to its customers.