In reply to Rob Exile Ward:
How low are you intending to camp each night and how many nights do you expect to consecutively bivvy? More than one night of rain in a bivvy bag I find unpleasant unless you have a tarp or are lucky with natural shelter. If mostly fairly low and more than 3 days in a row you might find such a warm down bag a bit claggy rather than cosy especially if you are going straight from walking to bedding down for the night. It is manageable if you can sleep outside the bivvy bag every now and again. I would be inclined to go a bit lighter and as suggested above have a dry change of light clothes you usually keep with the bag (don't take the stuffsac, put it all in a small roll top liner you can squish the air out of with your arse as you pack. If you are finding the moisture manageable you may even want to pack the bivi and sleeping bag together like this after a night where the outer is dry, saves time and you can deploy everything easily if it is wet when you bivvy again). You can air these garments out in a mesh pockets on fine and dry days. Even if you are planning the odd higher night I would tend to go lighter and suck it up rather than be too warm on any link up days you might have. Most of the modern quality bags have water resistant down now if you do decide to buy something lighter you may not need to think of a synthetic to allow for any multi day wetting.
I tend to sleep "warm" and would probably be happy with something quite light but it is a personal thing.