Unless you have the metabolism of a polar bear, I don't see this working out. I have used my parka over my bag, not in it and that works fairly well, but not to compensate for a 25 degree temp differential. -10C is +14. I do not see how a +40 bag and parka will work at that temperature differential unless you have a parka like mine (or similar) which is a TNF Baltoro (new model renamed Himalayan) which is a $600 down parka designed for sub zero temps. I have used a -5C (+23F) bag in temps down to around -10C with an MEC overbag or my parka on top wearing Capilene midweight base layer top and bottom and been warm and I am a cold sleeper. My parka weighs as much or more as a lot of sleeping bags.

I've never used a quilt, so I will defer to those who have. Ray Jardine makes inexpensive quilt kits and he has used them for years in cold weather. All the info is on his website (www.ray-way.com). I think a few of the members here may have one.