I have always used white gas (MSR Whisperlite) for cooking and melting snow, which does me well in freezing temps (after priming). I recently got an MSR Firefly which takes the isobutane canisters and, I am told, does not run so well below freezing. I would like to use it on some alpine trips to save some space and weight, possibly during cold conditions. Is sleeping with the canister in the sleeping bag enough to ensure performance of the stove in the morning during colder temps?

Just used a MSR Reactor for 3 months in Pakistan and was by far the best stove I have ever had. Used it in the mornings sleeping 3 nights at 7,000 m and yes worked better if you kept the canisters warm and have the blended propane/isobutane cans.

Have fun

http://seattlebackpackersmagazine.com/stoves-for-cold-weather/