Although I am by no means an expert, I do have a little knowledge of polar lows as they are part of my dissertation. There have been quite a few comments about the 500 mb temperatures not being cold enough, with -40 being quoted several times. This is a good guide temperature, but polar lows strengthen mainly due to temperature contrast between the surface and aloft. With 'warm' seas, it is not a requirement to have a 500 mb temperature of -40.
In regards wind speeds, there is no strict definition, although 15 m/s is commonly used.