R screams if he's too hot so i doubt he'd tolerate a snowsuit in the sling. He has a fleece one for the car, and often yells till his blanket is taken off! I'm very south and it's about 10* here, things might change if/when the temperature drops.
In the buggy, he has a blanket under, then 2 thin layers plus coat, hat and bootees, then a blanket over but I've just bought a cosytoes type thing. We've used the buggy a grand total of 3 times, though...