So IMO a 5" loft (both layers) semi-rectangular bag will not be as warm as a hooded/neck collared mummy of the same loft and shape. Unless of course some means of compensating for head and neck insulation is used.
How much more so, then, with a hood-less quilt, which is far more prone to developing drafts along the side if you move in your sleep or when you try to turn over? And even more so on the ground and with a more narrow quilt? The total body "sealing" around head and neck and sides is a considerable contributor to the warmth rating of any bag of given loft. Same thickness of top layer does not guarantee same warmth, and in my experience the difference is easily noted.
In my Ultra 20 in a hammock, I have done an extremely windy 18* OK with adequate clothing, including my hooded 14 oz PG jacket and 8 oz PG pants. Another snowy night in the mid to hi 20s, and using the same clothes/quilt, I woke up middle of the night a tad cool on top. This time I added my Marmot down hood ( well over 2.5" thick measuring one layer) and quickly became toasty warm and stayed that way. Another time I made it with this quilt and clothing for most of the night down to about 11*F, though I had to get on my side with bag pulled mostly over my head. But even with that, I was too cool to stay asleep by about 0430, so I had to switch to a thicker bag with hood, then I was fine. I wish I had thought to try my Marmot hood before I ditched the GL Ultra 20.