Robert: It all depends on your metabolism. Fortunately I am amongst the worst-heated in the world, so if you take what I pack, you won't freeze!
So, for me, I use a full-length Thermarest LE with 18mm CCF MEC purple Evazolite pad under that. I take the summer weight -5C Kluane down bag, with a snythetic overbag, wearing balaclava, pile long underwear and socks, and contact gloves. That takes me to -10C in still air inside a tent. I have been known to lay down a permanent aluminized space blanket ('all weather blanket') under the tent to keep it from freezing into the snow under body heat.
I once left the T-rest LE behind and froze!
In similar conditions (-12C) I have used a Bibler I-tent, same down bag, LE, foam sitz pad layed wide so my hands don't freeze when they fall off the LE, dual layer down pants, down jacket, MEC backcountry quallofil booties, balaclava, contact gloves. This system is more compact. However, if you get the down jacket damp during day use you are in trouble! So, I mostly use it if I am camping out at something really tame, like an arts festival with heated buildings around.
Last fall (August!) I bought a Thermarest Ultralight Women's Extra Long 4 pad, 740gr, which is R4.1, so in theory as warm as the 2001 1300gr LE, and the Women's pad is trimmed narrower at the feet so it will fit inside my bivisac for 'camping' on the back deck of Alaska ferries!
Try your gear outside in the same conditions as you expect on your trip. We all try out our stuff in our back yards at -40C to see if it really works.
If you don't have cold weather, ask to get access to a walk-in freezer at a big grocery store. Make sure they show you how to open the door from the inside! Then try out your gear. I would skip lunch or dinner before doing the test to simulate low blood sugar if you are in an emergency situation. As well, 'cold soak' your gear by leaving it in the freezer overnight so it is at -25C, which is about as cold as industrial freezers get---that is what MEC does in Vancouver.
My ration for winter eating is 350gr of cookies per day, first ingredient 'chocolate'!
I am only just experimenting with vapour barrier clothes and can't reccommend them as of yet. They show some promise as they do keep your insulation dry. See BPL's articles on I think Ryan Jordan's Icebox hike and see what he used. Your MEC overbag if it has the XCR membrane in it, is practically an insulated VB already, so you could try that. I have had down bags collapse due to body vapour overnight at -12C in August, I had on primaloft clothing inside the down bag....
We use the 'Mickey Mouse' Vapour Barrier boots here for dog-mushing and winter-biking, but I don't like wearing them all day if I can avoid it as my feet feel vulnerably damp-saturated...
PS: Designate a 'pee bottle' which must feel different in the dark from your water bottle. This saves enormous energy lost getting out of your bag....