For a better context, I typically use Schwalbe Marathon Racers on the recumbent which have a very shallow inverted tread and relatively flexible sidewalls whereas the Marathon Winter tires have stiff sidewalls and v-shaped knobs as well as studs.
Late last night, I stopped by the supermarket with the recumbent and I had an opportunity to test out the bike in varing conditions. Some of the side streets had remained unplowed such that there was 3 to 4 inches of snow and slush with little ice. Just based on my observations, the studs had virtually zero effect. Essentially I was riding on 42-46 mm knobbie tires through loose snow and slush. I managed to make it through with my feet relatively dry, but something wider -- more surface area as well as lower tire pressures -- would have been better. On streets that had been plowed, there was sections of hard packed snow/ice, opaque/black ice, and pavement. With regards to tire traction, distinguishing between the three was very difficult. Besides any visual clues, the tires produced different sounds on the three surfaces which is how I determined what I was riding on.
In short, if you're riding on (potentially) ice-covered surfaces the studs make a big difference. If the surface is simply covered in loose snow/slush, these are expensive knobby tires.
While I do not have a true answer to Jim's questions, I have a bud who picked up a set of studded tires for his mountain bike but inexplicably -- JP has plenty of free time -- has yet to put them on the bike. I'm hoping that JP plops them on and produces a comparison of wide knobbies versus studded tires. He picked up a set of Nokians but unfortunately the model as well as his present set of tires escapes my memory. I'll be sure to post something here when it happens.