FWIW, the coldest I've slept outdoors is 20F, and every time it got that cold I was happy to have a mummy.
I too found that for me a mummy bag has worked out better than a quilt. If I use a quilt it needs to be wide or have some means of keeping it in place and the draft out (straps, skirt, etc). As a result it winds up being as heavy (or heavier) than my 17 ounce Mountain Hardwear Phantom 45. When I want it to be like a quilt I zip it open and use it like one. When I want maximum warmth I zip it up and pull the hood drawstring to leave only a small opening to breathe through. Despite being a nominal 45 F bag it has served me well down into the teens F.
I have in recent years switched to using a bivy on most trips. That has me wondering if a quilt might work well with a bivy, since the bivy would keep the edges tucked and sealed when in the bivy. I have not bothered to experiment with that though since i love my Phantom 45 and there just isn't much weight to be saved when it is just over a pound.
Also I wonder why you would use a full closed liner with a quilt. Wouldn't an open bottomed liner have any of the same advantages or disadvantages the quilt has? That or an outer cover like a bivy.
BTW, the bivies I am referring to are not the tiny tent type withe sturdy materials, hoops, and what not. I prefer the ones that are just an envelope (5-7 ounces). If I am going to go with a bivy that is a lot heavier than that I will go to a regular light weight tent.
I find a setup like I use infinitely variable. I can sleep on top of the whole rig. Then as it gets cooler and breezier can progressively, sleep under the zipped open sleeping bag (maybe with an arm or leg out), get in and zip the bag, climb in the bivy, pull up he bag's hood, and then draw the drawstring tight.
I also adjust what I wear in the bag and sometimes pile any extra clothes on top of me for extra warmth. I also find a pair of very warm, dry, loose fitting socks a must in cold weather.
