My sleep system for colder weather is kind of odd since it's partially based on a sometimes accompanying intimate partner (spouse).
I have EE Rev X 40 degree quilt, and she has about a 30 degree quilt, but we also have the Exped Dream Walker Duo 400, which does have the neck opening*. The double quilt is not a very warm quilt, but i feel we get some benefit from having a double over quilt as i'm a much warmer sleeper than her and some of my warmth is trapped for her as well, maybe not directly (since her and my single quilt each is to some extent shielding her directly from my warmth) but in slowing down overall conductive energy loss.
We have tried using the dream walker quilt in it's "poncho" like form, but i'm kind of paranoid that it will get ripped, overly dirty, or what not, much easier than using it purely as an over quilt.
*(i realize that three separate quilts increase the overall weight of said system, however, the adaptability is awesome and i think worth it. She doesn't always come with, and when she doesn't i can either just take my single quilt or hers if it's colder, or sometimes she can take my quilt and i just take the double one since i'm such a warm sleeper usually, etc. It can be mixed and matched in various ways, which i really like. Also part of why i developed this system was because of the cost, all said quilt were relatively inexpensive, and about equivalent in price to a much colder weather, high down content double quilt such as Zpacks sells).