Re: 32-bit vs 64-bit
64 bits is irrelevant for SIMD. Nothing stops you from having 64 bit, 128 bit, 256 bit even 1024 bit SIMD running in 32 bit mode, any more than 32 bit CPUs were unable to use 64 bit or 80 bit wide FP.
There are real benefits to 64 bits however, even with only 1GB of RAM like the iPhone 5S has, which is shown in the performance comparison between the 5S running identical apps compiled for 32 bit and 64 bit. For certain apps the difference is exaggerated, because they make use of new ARMv8 instructions, but 10% or more is not unusual for real world gains. Not world shaking, but worth doing since it comes without any additional hit on the battery.
Implementing 64 bits before it is absolutely required is also helpful becauase it gets developers building 64 bit apps now, rather than waiting until they're needed, and will allow Apple to phase out the 32 bit instruction set sooner rather than later. Since Apple designs its own CPU, it will be able to completely dump the 32 bit instruction set sooner, which will make their 64 bit CPUs cheaper and faster.
They may do this with the upcoming A8, since it could run 32 bit code via translation. That would slow it down when running 32 bit code, but it should still be faster than a 32 bit A6 when doing so, and most developers have already updated apps that performance sensitive in any way to build a 64 bit version.