Of course the developers seemed excited, Apple wouldn’t have allowed them on stage otherwise. They’re excited about the GPU though, which has little to do with the width of the CPUs instruction set. There are some apps that can benefit, but they are a minority, and getting double the speed from them is also unlikely unless you’re doing atypical things with your phone.
Video encode/decode is typically handled by the GPU, so won’t see much benefit. Photography could benefit, but it’s going to require specific optimizations and may not matter since most of the stuff you do on the phone is pretty lightweight. It’s not like you’re running Photoshop on your phone. Even then, most photos are in 24 bit colorspace so it’s hard to squeeze benefit from a jump to 64 bits.
The biggest winner is probably cryptography. It’s math heavy and deals with very large numbers routinely.
