I can tell you that 6 cinder blocks in the trunk of my 400e made the wheel spin stop in the rain.
So for the snow I got 6 bags of tube sand, 4 for the TD and 2 for the 400e. Right now all 6 are still in the TD... once we have significant snow I will slap my studded snow tires on the TD, which started today at 10* without being plugged in
So I can safely conclude that extra weight will help the w124 with traction. Dunno about the 300e but in the heavy engine 124's it helps.