The answer is "nobody knows". Well, not precisely anyway. Most of the canister manufacturers won't publish exact percentages.

From what I've read, MSR is generally a good one for colder weather. Coleman and Snow Peak less so.

HOWEVER, if you're talking about temps below 15F, then forget any brand of canister with an upright canister stove. If, as one poster suggested, you're willing to buy a more specialized gas stove, one with a remote tank (i.e. the burner doesn't connect directly to the tank but rather is connected remotely via a fuel hose) and a pre-heat loop, you can go colder, at least down to 0F, maybe down to -5F. If you saw my post on the Coleman Xtreme, that's worth looking into. The Coleman Xtreme uses a higher propane content fuel and is rated down -4F, although I've seen posts of people using it down to -10F.

If you're thinking of just using a regular canister stove and just getting a better brand of gas, for those temperatures that won't work. You have to either get a specialized gas stove or switch to liquid fuel.

Liquid fuel, by the way, will work at far lower temperatures than any blended fuel gas stove. Arctic/Antarctic expeditions to this day use liquid fueled stoves. If you're heading out in really seriously low temps, liquid fuel is the way to go.

HJ