There are bunch of threads discussing sleeping pads, so you can check them out but here's my experience. Keep in mind that I assume you mean "real" winter, and not it may drop to below 32*F (0*C) a couple of times...so let's say nighttime temps of 5*F (-15*C).
1 x Thermarest z-lite - not recommended
2 x Thermarest z-lite - doable, but not recommended
1 x Ridgerest - doable, but not recommended
1 x Downmat 7 short, pack under legs - warm all night long baby!
1 x Montbell 90, 1 x GG Nightlite on top, pack under legs - good combo at a nice weight - will keep you warm.
1 x Stephensons DAM (custom 48" length), 1 x GG thinlite underneath, pack under legs - warm and toasty!
1 x Downmat 7 short, 40 below tent floor underneath, pack under legs - Just used this combo and I don't think you can get cold at any temp.
OK, so I hope that helps you or anyone else out there. For simple overnighters, you can obviously get away with less because 1 bad night won't kill you. But after a few days of bad sleep, it will take a toll - especially in the winter. I use a DM7 short almost always because it is so warm...it helps me push my versalite to much lower temps. If you want to shave more weight, the torso inflatable with a CCF is a nice combo. My montbell 90 and GG nightlite weigh about 13 oz and has my seal of approval for winter use. :)
I should add that if you plan on using your torsolite (a BPL one, right?), you could add a CCF on top and probably be good. Testing it is the key in the end.