I've been experimenting with a sleep system based on a lightweight down bag and a synthetic quilt over it when the need arises.
I use a Rab Quantum Top Bag, the one with the Pertex Quantum base. I put my Thermarest Ridge Rest, cut to fit, inside the Top Bag as recommended. This works, and cuts out any draughts. I usually wear a wool beanie and wool sox too.
When it is warm, I'm in this and a hiking shirt/ shorts- when cooler, I wear Icebreaker wool long johns and a Icebreaker Mondo Zip wool top. This does me down to about 5- 10 degrees C (40-50 F).
Cooler nights than this (due to season or altitude) see me in my Patagonia Micro Puff jersey, and I also take my MLD 2 Thirds XP Quilt made of Momentum and Climashield XP. The lowest I have been down to was 0C (32F) this weekend, and I wasn't wearing the Patagonia, I only had the quilt over my legs and mid torso.
I sleep in a SMD Serenity Net Tent inside my Golite Shangri-La 3 pitched high to avoid condensation. Under me, I always put a full length Pashmina Shawl for insulation and comfort.
It's a flexible system, and I could probably take it lower than I have, but generally don't hike overnight much in the winter. If I did, I'd use my heavy 3/4 season Paddy Palin down bag instead of the Top Bag and Quilt. I sleep warm, and my temperature and the outside temperature changes a lot during the night, and I've never had a comfy night in my old 3/4 season bag- I always woke up sweating and clammy- even with venting. YMMV.
When it's warmer, I can sleep under the quilt on top of the Top Bag, and crawl in when the night gets cooler.
I've still got layer options with the wind jacket and Pertex wind pants as an option if things get really cold, but so far, I haven't had to use them as well as everything else.
I haven't found putting the quilt over the down bag impairs performance due to loft suppression, and I always have the option of wearing the quilt in the bag if need be.
As far as storage goes- I live in a small apartment in Barcelona, and store most of my gear in the rucksack, and the sleeping bag in the storage bag it came with, which doesn't take up much room at all.
Weight of Top Bag/ Quilt combo- 650g
Weight of 3/4 season bag- 1.5kg (it's an oldie, but a goodie)
I learned how to do all this from reading this site, so thanks to everyone.
I hope all this helps.