That MS has a working x86 compatibility layer makes me think that Apple could well have something like this too. If the new chips have performance comparable with Intel's, even a 5x performance hit for "legacy" apps might not be catastrophal, since developers in the Apple ecosystem are usually fast to update their stuff and cater to the demands of the early adopters with lots of money.
Apple has gone the emulation route with the PPC/x86 transition before. With their tight grip on the ecosystem, including the development tool chain, I think most software will be updated very quickly.
This is quite different from the situation with Windows software, most of which feels like the Devs hate the it, the platform and themselves.
