The pac boots are good for -10 or so if you are working. They are not when you quit. The insulated workboots just mean that you can spend 1/2 hour - 1hr getting cold gradually instead of having your feet freeze almost instantly when you walk out. If it is going to be + or - temps, you probably want some rubber on the bottom so they don't get cold and wet. Cold weather boots work when it warms up, but it doesn't work the other way around.
The best way to get the most out of your boots is to wear long johns under pants and to wear a hat. If it is going to be bad cold all day, and you will be in it, you might also add a pair of coveralls over the top of all of it. They keep a lot of warmth you lose out of your legs and spread it over your core.
The ultimate boot for cold weather is the rubber bunny boots, black ones for down to -teens and the white ones when it gets colder than that. They are heavy, but are also water tight, and if water comes in over the top, you can dump it out and keep going, as it doesn't soak into the insulation.