Well there are a lot of variables.
First, you need the exact temperature. Then humidity. Then how many shots your taking, as the fps will go down consecutively after each shot. Then math it all, and you'll have your answer
But seriously, most things that run off propane don't work for more than 3 or 4 shots below 0 degrees. They're optimal temperature range is above 20 degrees.
CO2 will have more power at the same temperature as propane, specifically because it's a more powerful and stable gas. However it suffers from the same cooldown effects. You still won't get more than a few rounds off before the valves freeze open.
T7, I'll add on to your point;
It depends on the gun. GBB pistols work like AEG's for the most part where differing weights of ammo have the same energy. However in gas rifles with longer barrels, heavier ammo typically has more energy. For example my PDW shoots 380fps on .20g BBs, but on .28g BBs is shoots the equivalent of 405fps on .20g.
But it still doesn't work worth a damn at 0 degrees