I would consider how much you may sweat in your clothes
and how much mosture you may put into your parka if
you must use it on the move. The only thing that you have
to dry out your clothes and keep you warm in the worst
case is your sleeping bag/tent combo.
I also froze my butt off with the darkstar, climbing in the north east, on mount washington etc. It is rated down to -40f degrees. I gave up on it and bought the best bag(expensive) I could find, to my knowledge. Stevenson's warmlite.
It is a down bag, I had a light down belay jacket on a another trip on the washington range, got the down jacket
soaking wet from body sweat. Also my clothes wet, this down
bag being way over rated for the temp enabled me to dry
my clothes out in about 30min but not my down belay jacket, it wouldn't dry out.
Opinion, I would rather have the warmest bag possible
and a good tent and not have bivy sack. The sack will
hold mosture in the bag and slowly kill warmth, if your
like me and sweat a lot this could kill you on a long
trip. If you don't sweat then your case is different. If
I used a bivy sack I wouldn't want to have to use it
for too long as my sleeping bag would get wet. Note,
if you need to dry out your clothes you need to move the
mosture out of your skin and clothes, to the bag and then out of the bag into the air inside of the tent. Breathablity is really important, and if your dehydrated, AND frozen AND all your clothes are soaking wet. Then a extra amount of down in a breathable sleeping bag system is what you need to dry yourself out and after to get warm.
Hope this helps.
Dan