ARM is currently about around a step, give or take half a step, behind x86-64 in raw performance, but they are even closing that gap. ARMv8 looks really promising in the architecture-agnostic realm of server farms, where x86-64 uses wads more power to do the same work: if you can switch and cut out three-quarters of your A/C bill, why not? Most server software can be recompiled in a trice.
I am not clear on the exact details, but right now, i7 and Xeon have a pretty good edge on ARM, by the clock. But that could change. No one could have imagined Mac-on-ARM ten years ago, but they have made huge progress since then. They could start to move ahead very soon.
The biggest advantage for Apple is that they can map out their own CPUs to make OS X run optimally. And the price-per-mflop for ARM is, so far, much better than for x86. The only places ARM would not be serviceable right now are as a replacement for the Xeons in the Mac Pro, and the i7s in the top-end MacBook Pro, the really low-volume lines. But that could change too.
If Apple makes a transition, they will do it even more seamlessly than the last transition. No ARM Mac will be without some really good form of native x86-64 operation, be it x86 cores within the CPU or an amazing code-morphing coprocessor system, for many years to come. Even Bootcamp will still be possible, though it will be at least a little different.
But this is not going to happen soon. As in 2-years=soon. I suspect when they do this, they will get it right this time.