They said that they implemented x86 ISA memory handling instructions, that substantially sped up the emulation. I don't remember exactly which now, but they explained this all in a WWDC video about the emulation.
There's also the CFINV instruction (architectural, part of FEAT_FLAGM), which helps with emulating the x86-64 CMP instruction.
"Apple created a chip which is not an X86! Its awesome! And the best thing about it is ... it does TSO does like an X86! Isn't that great?"
I think the last time I ran amd64 on my mac was months ago, a game.