In reply to huckleberry:
I used to use a rab 500, no zip, 500g down, 500g of cover, total weight 1000g. That was 1987, lighter materials and loftier downs are now available. Bivied up to 4000m always over warm except only just warm camping below the Cap with a thin mat.
So your wooldown options look on the heavy side.
I have now got mountain hardwear lamina bags, fully synthetic, with a short centre zip and double puller. Good for drinking a brew , staying tied on and having a slash without getting out too much. Weighs less than a kilo, probably lighter synthetics available .
