I also have a WM Super Antelope 5-deg bag and get cold at 20-deg. Goes to show that there is a lot of individual differences in what is "warm" when we sleep. I personally do not like the way the WM "draft collar" works- it is not a real draft collar, rather a double draw-tight system. This saves weight but I do not find it as warm as my old Marmot Agille bag (which was unfortunately stolen) that had a true draft collar. My WM bag is a short, but not proportionately narrower, so I really have trouble getting it warmed up and keeping it from being drafty. Be careful what you wish for with respect to roominess. For a winter bag you may also want a little extra length so you can stuff clothes down at the foot.

I have an old snap-off hood from a down jacket that I wear inside the bag when it gets cold- it really adds warmth for a few ounces.

For long term winter camping (say 10+ days out) I think I would prefer a synthetic bag, particularly in high humidity conditions (such as lots of time in a snow cave). Ice crystals build up inside a down bag and reduce loft and synthetic bags dry faster. I have not done winter camping for a while and the advise for synthetic bags in winter may not be current thought. We used to recommend a double bag- light tight-fitting 45 degree down inside a roomy 20-deg synthetic. The ice crystals tend to build up on the outer inch or so of the bag.