I got my MY12 N55 PPK'd soon after it arrived from Germany.
Small bump in power, a bit more torque and better throttle response - as many others have also reported.
Maybe there is a little more turbo noise due to the slight increase in boost, which was something I remember noticing. Also perhaps slightly louder on the exhaust note when on the gas, but not by very much compared to the stock N55.
Final note: I was mashing the pedal more after doing it as I wanted to see the difference more prominently, hence noise would have definitely increased!