BTW, have you considered using vapor barriers as well?
This gives a tremendous amount of flexibility and may change the way you think about a bivy system.
I recently started experimenting with them for sleeping purposes. Being as I tour I really work up a sweat during the day. This pretty much mandated I carry a second pair of cloathing for night.
However if you use a vapor barrier you needn't worry about wet clothes or condensation in your bag. What's more you don't have to have a breatheable bivy.
You can if you like use a vapor barrier pant and shirt or simply a vapor barrier sleeping bag liner.
The advantage of the vapor barrier pant and shirt is the ability to use it for extra warmth during the day if you need it in an emergency.
BTW, lightweight plastic grocery bags make excellent vapor barriers for socks. Just use you lightest sock inside and a heavier sock outside.
PVC rain pants seem to make a great vapor barrier legging. Since I only wear a single layer pant during the day I just slip them on at the end of the day when I'm cooling off anyway and then slide right into the bag.
Ideally you want to have only one very thin layer between yourself and the vapor barrier, i.e. ultralight merino wool or polypro (works but smelly). The thin inner layer is for comfort. Some production vapor barriers include a comfortable cloath liner.
So, at the end of your day when you're cooling down at some point you quickly take off your outer layers and put the vapor barrier shirt on just a light layer away from your skin.
The vapor barrier alone should get you another 5-15 degrees more out of your bag. What's more using an outer non-breathing bivy… i.e. mylar, should give even more warmth.