I have 2 bags of 40 lbs of kitty litter over each wheel (off to the sides of the trunk). So 80lbs per wheel. And the best thing is, I can use the kitty litter for my cats come spring time!
I also have winter/snow tires on a 2nd set of rims that I use. That makes the biggest difference of all, I think.
On Monday was the first test with the kitty litter in the trunk this year. Drove 350km in 5 hours because of a bad snow storm. That was a boring drive...