Couple of things came to mind:

- Heavy duty down bags for cold winter conditions (such as the -32C mentioned above) are big, bulky, heavy and cost serious money. If remotely possible, try to get a guesstimate on how warm you really need it to be. Then add another -5C to give yourself some margin and spend whatever is needed to get there.

- There are few things you can do to make the bag warmer. Use a good (inflatable) sleeping pad. Adding a liner inside the bag will help. Silk is the preferred material here, if you can justify the cost.

- If you're staying in tent for several really cold nights in a row, your main problem will be keeping your gear dry. Some people prefer taking their (wet) touring clothes inside the bag to dry during the night, some say that only results in slightly drier clothes, wet sleeping bag and a miserable night's sleep. A down bag is next to useless when wet and takes a long time to dry even with proper heating (which you won't have in a tent).

- As you have camped in -7C, odds are you have a reasonably warm bag already. One thing you might try is buying a large down bag rated to slightly lower temps and put your old bag inside the new one. You will not get the same insulation per weight ratio as with a good single winter bag, but the cost will be considerably less.

--J