Lots of good feedback so far.
Some points: I think most people consider Nunatak quilts to be representative of the category, ie narrow-cut mummy shaped. If you look at the similarities and differences between mummy bags and quilts, it's very hard to see a significant advantage in terms of weight or other advantage to a quilt... other than personal preference. (In many of the instances we tend to compare... at the fringes, absolutely some differences. To me, the point of a quilt is to absolutely minimize weight. The only way to do that at a certain point is to use less material. The only way to really make a quilt significantly more weight efficient...ie, lighter... than a bag is to make a really skinny-cut quilt. More to follow...) And personal preference is a very significant factor in finding your happy sleep space.
Nunatak's Arc Specialist has a girth of 55" and footbox of 38", mass of 16-17oz (depending on shell). Add an ounce for longer length (as suggested for shrugging into if cold). You're looking at 17-18 ounces for the quilt. Nunatak's down balaclava adds 4 oz. By comparison, Western's Summerlite weighs 19oz, for a 59" girth and footbox of 38", but it comes with a hood and zipper. The Nunatak, even without considering the weight of a bivy, is an ounce lighter for the quilt itself, but about even or a bit heavier if you consider head insulation (since you can lose over 50% of your heat through your head, an important consideration).
Of course, if you have a down hood separate from the sleeping bag, you can then also use the hood with your jacket around camp. I don't find that particularly useful in 3-season use, because a poofy down hood is simply too warm for moving around camp. But for straight-up winter camping I could see it handy. In fact, I designed my last down jacket/quilt to work with such a system. But I found that I stayed warmer with less fuss with a more conventional system.
If you considered the Ghost instead, with a girth of 46" and footbox of 34", then it only weighs 14-16 oz, + 2oz for going up a size in length, versus the full-girth, zippered, hooded sleeping bag.
If the point of a quilt is to use less materials to get a lighter product, then it doesn't make sense to buy a longer quilt and a heavier bag. If you got a Ghost in medium, let's call it 15 oz (split diff. between the two lightest shell materials). Most people will still add some kind of head insulation. And from what I can tell, most people who use quilts do also carry a bivy... but assume no bivy. After adding some kind of balaclava, you've added at least 2 ounces to your sleep system... at which point, you're still 2 ounces lighter than a Summerlite. But the Summerlite is roomier and will therefore be easier to layer under, plus you won't have any drafts to deal with.
The quilt/bag I designed and used for my last trip was a finished girth of about 56" with a footbox of 34". I found that the size of the footbox kept my size 9 feet kind of locked into position in the footbox... in other words, when I rolled over the quilt would roll over with me, because it stayed in alignment with my feet. To roll over without taking the quilt with me, I had to first pull my feet up out of the footbox. I also found that the 56" girth was not quite adequate for side sleeping... if I rolled over, an edge would pull up quite easily and create a draft. Yes, I was usually able to cinch that closed. But in a sleeping bag, I just roll over with the bag. It never comes out of position, it doesn't restrict me in any way. I actually have more room inside the bag to move around than I would under a quilt... which makes sense, since quilts are narrower cut and have to be locked into position on your pad to keep the heat sealed in and the drafts minimized or out.
Regarding layering under a quilt versus a bag, if you buy a larger-girth quilt to allow for layering, you might as well buy a bag. My Summerlite at 59" allows plenty of room for my relatively stocky build and a down jacket or vest if needed.
In short, if you buy the absolute smallest quilt possible and don't carry any head insulation or a bivy, you can get a system that weighs a quarter-pound less than a full-on sleeping bag... simply because you're using less material. However, the weight of a #3 zipper (full-length, about 0.75oz IIRC) and hood (about 2oz) can be a few ounces well worth it... perhaps even more efficient in terms of weight. And in terms of versatility, I regularly use my Summerlite as a quilt, but you can never use a quilt like a sleeping bag... ie I can drape a sleeping bag over me just as I can with a quilt, but I can also zip the sleeping bag up to maximize warmth when necessary, stick my head in the hood.