Here are a few random thoughts for you.
There are some guidelines in the Recoiled book for difficult travel, in chapter 8. They say difficult terrain is 1/2 movement, for instance. So, I could see snow in normal terrain being equal to 2/3 movement (or minus 10 feet). Walking in the snow over tough ground could be really slow (1/3 or something).
There are also some guidelines for taking damage from the cold weather if you aren't dressed for it.
I can see a penalty to Listen if you have a scarf wrapped around your ears, but I don't follow you regarding a penalty to spot. Now, if someone is dressed in white and hiding in the snow, then that is covered by camoflage just like someone dressed in brown and green hiding in the trees. Other than that, I don't think snow would have a special penalty to spot. Likewise, if your ears are uncovered then listen should work fine as well.
Regarding hampered movement, I've never seen anything about that. However, you could use the Sleight of Hand skill as a guide. They give a +2 bonus to concealing a weapon if your clothing is bulky. It would make sense that you would get a similar -2 penalty to your initiative when drawing a weapon in that situation.
Max