I think 30°F would be very optimistic.
I did have a sewn through TQ from Tewa that had the same 5oz fill. Unfortunately for me the first night I used it temps dropped to freezing. Even with all my clothes, rain gear etc I was still cold, actually re-write that, I was freezing and shivered until daybreak from about 1pm the night before. Those sewn through quilts are realy only a summer quilt, exactly as they are made for.
I think getting an extra 20°F out of a quilt might be possible but few would get there in comfort.
The amount of extra clothing you'd need and the weight of a liner would be heaps more than a correctly rated quilt would weigh. It's amazing the extra warmth 3-4oz down makes to a TQ, oh and baffles help to.