How do microcontrollers achieve > 1 MIPS/MHz performance?
DMIPS and MIPS are not the same. DMIPS stands for "Dhrystone MIPS", and it's a standard for comparing different microcontrollers/microprocessors' performances across different instruction sets. The standard is not so new anymore(*), so what the reference processor could do at 1 DMIPS/MHz in 1 second may take a more advanced controller 670 ms at the same clock speed, just because it will do some actions in 1 instruction whereas the other controller may need 2 instructions, or an instruction requiring more machine cycles.  

A 16-bit architecture, for instance, will typically have a better Dhrystone performance than an 8-bit architecture, and a 32-bit, like the one you're referring to,  even more.



(*) m.Alin refers to the more recent CoreMark  benchmark. Note that this is written in C, so actually you're also including the compiler's performance  in the result (just like Dhrystone, BTW).