As per the research of R. Nisley, we've learned in the past year or two that the warmth of a down sleeping bag isn't entirely correlated with loft. Nisley has shown down can be compressed moderately (ie. 2-3x) and not really lose it's effectiveness. Down compressed 2x might only have half the loft, but since it's now a higher density, it's nearly twice as warm per inch of loft, so the total warmth is hardly affected. This holds true until a surprisingly high amount of compression, so even down compressed 3-4 times is still going to have most of it's uncompressed insulating value. It's true that it's most efficient to have down not compressed at all, but it's almost as good to have it moderately compressed.
So one sleeping bag with 3" of loft could be much warmer than another sleeping bag with 3" of loft, if the one sleeping bag has a lot more down packed in there. BPL's position statement on loft and warmth is quite outdated, and really not that useful for determining warmth
Ultimately warmth almost entirely comes down to how much down is in the bag, divided by the size of the bag. While it's not 100% perfect, simply looking at the amount and quality of down in the bag is the best way to make a quick judgement on it's warmth.
Before you can really compare how much down is in two sleeping bags/quilts, you need to consider how big the bag is. Zpacks sleeping bags fall in between quilts and normal bags, because they have a little more girth than a normal quilt, but still no hood like a sleeping bag. So to be just as warm as a normal quilt, they need a little more down (since it's spread over more area).
With that in mind, we can observe that Zpacks puts 7.3oz of high quality down in their regular sized 30F bag, while Katabatic Gear puts 9.5oz in their 30F Palisade quilt. Considering that the Zpacks bag spreads that down over more area (56" wide vs. 52" wide for the Palisade), we can conclude that the Katabatic 30F quilt is easily warmer than the Zpacks 30F bag.
If you expand these comparisons to include other quilts and brands, you'll see the Zpacks bags typically have less down than other options. The GoLite 3-Season quilt uses 12.5oz of down to get its 20F rating and Katabatic puts 12.4oz in it's 22F rated Alsek quilt, yet Zpacks puts only 10.5oz of down in their 20F 'bag' even though it divides that down over more surface area than the other two options.
I'm not saying this to bash Zpacks. I love their stuff and I think their quality and materials is awesome. I just mention in the hope that people will largely ignore temperature ratings and instead focus on how much down they are getting.