When winter camping, I nest a down bag inside of a larger synthetic bag with very good results.
The outer syn bag actually works better in the combined arrgt when it does on its own.
inner - rated 15 deg F down bag - a true 15 deg F for me
outer - rated 15 deg F syn bag - but really only good to about 25-30 deg F for me because it is a wide cut with lots of dead air space
I have used the combined set down to -5 deg F and feel I still have a tiny bit margin.
I think the key is that the bags fit together extremely well so that the inner down bag gets full loft and there is very little dead air space between the bags and around me.
Of course, nutrition, hydration and fatigue play huge roles as well since it is the heat from my metabolism that heats the air trapped by the bags