Not only valid ARCHs has to include arm64 but ARCHs does also. It should say armv7, armv7s, arm64.
When you change this setting the compiler will emit warnings for code that it believes not be 64-bit clean. You will almost certainly have dozens or even hundreds of warnings. If you've never seen any warnings for 64-bit issues then you've probably never turned on 64-bit.
Regarding whether apps run faster in 64-bit mode I don't know. Apple has seemed to claim that in general apps WILL run faster in 64-bit mode. All apps. Apparently the iOS libraries are present in both 32 and 64 bit versions on devices that can run in 64-bit mode. The 32-bit libraries are only loaded if some app needs them. If all apps on the device are in 64-bit mode then the 32-bit libraries will never need to load. And Apple says that would be a good thing.
For some reason building for 64-bit mode requires NOT building for iOS 5. So although I've fixed the code to compile clean for 64-bit mode for a few apps I haven't actually released an app that is compiled for 64-bit mode. Probably we'll see more and more 64-bit apps over the next six months to a year.