On the Arctic 1000, they each brought 17 oz of (lightly) insulated clothes in a pullover hoody and pull-on pants, coupling that with a 24 oz two-person quilt, which obviously couldn't have been too warm.
Sleeping bags are, without question, far more efficient at trapping heat than clothes, especially per ounce of weight. (think mitten philosophy)
So, on each trip, I believe it is important to decide whether to bring insulated clothing and/or a warmer sleeping bag. Where as you can wear the clothing and not have to be in the bag (moving in cold weather, in camp, after a river swim, etc), it is far less efficient for the weight than a warmer sleeping bag. However, one must be laying or sitting in a sleeping bag to use its warmth, meaning you can't be moving and probably not doing too much around camp, as you wouldn't be able to get too close to a fire or get up to do anything if sitting in a bag.
I see this conundrum as being similar to using some sort of stove (denatured alcohol or butane) or campfires to cook. If mileage is your priority, bring the insulated clothing, as it is far more versatile and can be worn while moving. Campfires require a lot of time searching for fuel (wood) and sitting around & tending them while waiting to cook. Stoves are much more straight-forward and time-efficient, although heavier, like the clothing option.
One other thought... Have you ever put a cold-weather glove inside a mitten shell and wear that system when your hands were cold? It doesn't keep your hands nearly as warm as a mitten! I think this is similar to the potential situation of being too heavily clothed inside a sleeping bag. There is no shared warmth (without significant production of heat, i.e., exertion) between body parts and throughout the bag because clothing isolates body heat instead of sharing it with the rest of the body (like a sleeping bag), so one becomes cold, even thought they are aptly insulated.
Here's what I wrote after a trip during the summer of 2010:Sleeping bags are not necessary for travel during summer in Alaska’s backcountry. Clothing applicable to the current weather conditions is sufficient for sleeping when combined with a light bivy bag, in the proper shelter. We used our clothing, coupled with 6.5 oz silnylon bivy bags, and slept on our inflated packrafts under a GoLite Hex 3. The packrafts fit well in the hexagonal teepee shelter, which seemingly holds heat well. We were able to sleep for a full 8+ hours, most of which was almost too warm. Being too warm can pose problematic, though, as one would become damp from sweat and then cool far too much to remain comfortable. Sleeping bags, however, provide a great amount of comfort and security, which can also be lighter and more practical than sleeping in clothing only. After wearing clothing all day, in all weather conditions, the clothing can be damp and sleep can be challenging. We found the system of bringing just enough clothes for the conditions and a slightly lighter sleeping bag than appropriate for the conditions was the most effective and lightest technique when used together, as a sleeping system. This is the same findings and methodology that many adventurers on light and extended backcountry trips have agreed upon and found to be most effective.
High mileage each day? More insulated clothing, lighter sleeping bag.
Not as mileage oriented or ounce-counting? You're on the wrong blog.