yes, im actually stellar at math...
Your first comparison isnt really fair. the WM is sewn through, but the arc edge is baffled. Because of this consturction, by default, the quilt is warmer. Likewise, the arc edge uses all the insulation on top of you, while the bag utilizes part of its down insulation underneath you, where it's crushed, resulting in very little insulation
Your second example speaks for itself. Why carry the same weight and lose 6 degrees of warmth? Another way to look at it is this...I'm going to have a down jacket of some sort anyways, since i'll be using it around camp before I sleep, and after I wake up. If i couple that with the 11 oz arc edge, I can push that bag down to the low 30's. All that for only 11oz. Where else am I going to find a bag that will be adequate in the low 30's and only weighing in at 11oz? You could get a WM summerlite but that weighs 19 oz. I'm shaving half a pound by making use of stuff I'll be putting in my backpack anyways.
And a quilt isn't rocket science. As a matter of fact I have an easier time getting in and out of one when compared to bags that utilize half zippers to shave weight.
The basic idea of the quilt is that it's utilizing the same amount of down fill, but giving you more warmth, since all the down is being place ontop and around you, as opposed to a bag which partially uses some of the down fill directly under your body, where it's crushed and worthless.
Read Dan's posts...they illustrate this perfectly
It's one of those things where you need to try it to understand. I wasn't a believer until I took a 19oz (9 ounces of down fill) quilt down to 22 degrees. A non quilt user may have taken a 20 degree WM ultralite, which weighs 29 ounces. That's a savings of more than a half a lb and I was still plenty warm.
Arguably, a WM summerlite weighs the same as my quilt, and with the same amount of down fill, but unless you can get all that down fill around the continuous baffles, and ontop of your torso, as opposed to under your back, it's not going to have the same level of warmth as my quilt.
Another point thats equally as important. Why do we focus so much about having awesome r-value mats? It's because bags don't insulate your back all that well (again, crushed down). So why not eliminate the back and have your sleeping mat pick up the slack? This is exactly the point with quilts
Lastly, I know you focus on EU ratings a lot, but IMO I feel paying attnetion to the down fill weight will you a better idea of the warmth. The weight number will be far more objective that any rating number a company posts.