In reply to lone:
I don't think a silk liner adds 5 degrees worth of warmth if that's what you're saying. I'd say just a couple of degrees. I use one, and it does make a difference, but for me, its more for comfort and keeping the expensive sleeping bag clean.
I have no experience of quilts, so can't really comment, but I too am not convinced and I'm not sure they will prove as flexible in use as a proper sleeping bag. Though I get the logic that the down under you is squashed, its only that immediately under you, and right up to the contact point the down will fill the void close to the body, keeping warmth in and minimising the space you need to heat up. I would want a warmer mat than usual if I were to risk one too.
I have a PHD Minim 500 for autumn, early spring and winter use, which is a fantastic quality bit of kit which will last me years and was worth the high investment, as I use it for probably 25-40 nights each year. I also have an Alpkit Pipedream summer bag, and have been impressed with the quality of that too, so their warmer range might be worth a look at (as well as Mountain Equipment and Rab of course).