I certainly wouldn't put anymore in. I have heard some people say it is not needed at all. This is my third winter with my 420SEL (we already have over 2 feet of snow here in Toronto)and I have done OK without them. I am planning on putting smaller ones in this year, maybe two 30 pound bags. Not too much extra weight, but just a little more. Also this is a good amount of sand, so if you ever have to pour some on the road for added traction you'll have enough. The most important thing about driving in wonter conditions is how you do it. That includes speed, slowing more for turns, etc, and sometimes knowing when it just isn't worth it to be out driving (whether all together, or just on a particularly bad road).
1995 E420 - retired
1986 420SEL - retired