When I made UL summer SBs for me and my wife I put 300 g of 800 loft down in them (total wt 550 g). Wearing a full set of thermal underwear, a fleece cap and bed-socks we have often taken them down to 0 C easily. Snuggled up to my wife with both SBs layered over the top of us (ie one SB on top of the other) we have been warm down to -7 C (19 F). It wasn't meant to get that low that night, but stuff happens... However, let it be said that I think my wife contributed greatly to my warmth that night, and v/v. The 50 mm Deluxe Therm-a-Rest (780 g) helped a lot too.
Emboldened by this, I made a 'double' over-quilt with a short foot pocket (a single layer of fabric) underneath. The foot pocket keeps the foot end of the over-quilt in place over our feet. I put 600 g of 800 loft down in this: 300 g apiece. If I layer this over my summer SB I reckon I have something like 600 g of down equivalence - without any heat from my wife.
With this over-quilt over our UL summer bags and full thermals, hats and socks, we are happy down to -10 C (14 F) without snuggling very close. If we really snuggled up ... not sure, but even lower.
But this is just with ordinary thermals on. If we put our light fleece ski trousers and our Cocoons on over the thermals ... well, we tried that once in the snow and were way too hot!
Would we go back to conventional SBs? Nope. Not any more. The quilts are far more flexible. With an adequate mat underneath, and that is *important*, quilts are no different from SBs.
Ah - one thing. If, in a burst of enthusiasm for all things UL, you make the quilt too narrow ... you have only yourself to blame. It will be a failure. You need to be very generous with the width - unless you sleep dead still on your back and never turn over in the night. Doesn't happen.
And another thing. For reasons of UL-fanaticism, many American quilts are made short and with no 'hood'. So the sleeper's head sticks out. Sometimes even their shoulders are a bit exposed. This is a a sure way to a cold night and is a pitiful 'economy'. My summer SBs all have hoods, and if it is cold we flick the hood over our heads. This does *not* impede our breathing at all. Claims of suffocating are made by those who have rarely tried this. Funny - no-one seems to have this problem with serious winter SBs with full hoods.
And another thing. When you go to a two-layer quilt there are unexpected bonuses. Your inner quilt will stay *dry*. Any condensation will occur in the outer SB. Pack them up separately of course. For the obvious reasons, the result is that you stay warmer even on long winter trips.