It might be that the Siri functionality is tied to the A5 chip. The A5 design is a lot larger than other dual core ARM A9 designs (even after accounting for the

It might be that the Siri functionality is tied to the A5 chip. The A5 design is a lot larger than other dual core ARM A9 designs (even after accounting for the larger GPU) and thus has lots of spare silicon to use for specialized circuits. It is possible that Apple added a custom DSP to aid speech recognition.