I use two pads all the time <img src="/images/graemlins/smile.gif" alt="" /> Extra comfort!
Prolite 4 on top with a RidgeRest Deluxe on the bottom.

Great info.

the sleeping bag could be part of a shelter system, rather than viewed as a stand alone. Perhaps you start with a lighter bag-say 20 degree capability, given that the ratings are assigned by marketers and have no objective meaning. The sleeping bag prevents conductive and convective, and maybe radiational heat loss on top of you, and its effectiveness is direc
tly related to thickness and trappe-air content. Under you, your butt compresses the insulation and markedly decreases its effectiveness. The ground is typically cold and wet, so you want a relatively non-compressible layer of insulation-foam pad, dry forest stuff, and a waterproof, reflective layer-space blanket, poncho, tarp.If it's colder, you can add a sleeping bag liner (down, fiberfill, fleece), a bivi sack cove (gore-tex, microfiber) or a poncho-poncho-liner combination. a tarp, or tent, or improvised shelter reduces convective loss, and keeps the rain-snow-sleet off your face. there are some interesting modular bag systems available that contain outer ba, inner bag, bivi sack combinations. The big agnes line of bags save some weight by putting no insulation on the bottom, assuming that youy will add a foam pad.
or you can do the canvas bedroll and wool blanket deal that the cowboys used. Unlike miracle fabrics, it won't diappear if you get it too hot, and will wear long enough to be part of your estate. heavy, tho

Cameron,
You are right; you must always use a pad or at least create some insulation between you and the ground. Otherwise the ratings mean nothing.
A tent will add probably about 10-15 degrees to the outside temp but this is a guess. But more then that it prevents the wind from becoming a factor.

I'll tell you from experience when it comes to keeping warm; body fat adds a lot. The leaner I am the more I am cold.

2 pads in winter is a good way to go, use closed cell foam on the bottom like a ridgerest.

Billy

Seven pounds isn't horrible when you look at the cost of the Colman. Lighter, stronger, cheaper- pick the one you need and the one you want, becuase you aren't getting all three.

When bags are rated, it is an unwriten rule that they are assuming you have on a proper base layer. (Maybe it is written, I've just never found it.) At 10, that means socks and a beanie, long johns and something around your torso at -10. It is only one and two season bags that they figure you are going to sleeping in your skivies in. If you aren't properly dressed, you need reduce the effectiveness of the bag. They are also expecting you to have it on a pad suitable for the weather.

Who's making a waterproof bag? You can get gortex bivy bags and such, that go outside your sleeping bag, but I can't think of single reason why you'd want one below zero. You want to be able to get your water vapor OUT of the bag, even if that means you are loosing X amount of calories, if the bag gets wet, you'll be loosing about 4X calories on the first night, and pooched every night after that until you have a chance to let it air out some place warm for a week or so. Once down gets wet, it is a major pain in the butt.

Others have answered the question about fill pretty well.