Ugh. People still spreading complete ignorant remarks towards 64-bit. All those things about "32-bit apps will have to be recompiled, wont be compatible" "the emulation layer that will make then run slower", maybe be applied to windows and x86_64 but may not be the case for the ARM 64bit architecture. Which apparently is more than just an extension and changes other things as well.
On the particular case of x86_64 processors, 32-bit binaries STILL run natively, x86_64 is just a superset that includes the older instructions. So, per se, there is no reduction in performance. When interacting with the OS, apps that run in 32-bit will need the OS to provide resources this way as well, and that-s the whole point of WoW64. Still that doesn't imply impacted performance, it means more stuff from the OS will have to be loaded in memory. So it only impacts performance when memory is limited. But since most people agree that the whole point of 64-bit is for "more RAM" (I dont agree with this), then be rest assured that it is not that much of a problem.
As for ARMv8 in 64-bit, read this http://www.realworldtech.com/arm64/ , then have an opinion.
There's also implications on how the OS works. Most android apps are developed in Java, which runs on a virtual machine (I said most because portions of an app can be developed in native code). And these Java apps? They're compiled on the spot. So if the people behind android transition to 64-bit on ARM, if they do it right, most apps will still run and not a "compatibility" layer, but on 64-bits. Java itself is already an extra layer anyway. That's why you can have android on x86 systems and still run the same apps, without developers having to do much at all.