These devices still don't have more than 4 GB of memory, let alone single applications pushing that limit, and it's not x86; the biggest win in moving to amd64 was the increased number of registers.
Which is not to say that there aren't others, but I don't think it's a clear-cut as for x86.
All of which is to say that I don't know. How would it help? Please explain, as I'm not that familiar with ARM hardware.
There are many improvements. To name one, arm 32bit has only 16 32-bit registers, while 64bit has 32 64-bit registers. That means instead of putting variables on stack, now you can put them in registers.
The biggest win in moving to 64-bit on ARM is the new 64-but instruction set (AARCH64) is modern and has improved performance over the previous instruction set. There are often 30-50% ir higher performance improvements. You do need the hardware though, and there's still very little 64-but cpu's out there for Android.
64 bit is supported. The Nexus 9 has a 64 bit chip, and they released an emulator for 64-bit Intel chips about a month ago. Motorola just didn't put one in the Nexus 6.
