Summer blends contain 2% butane, and winter blends contain 10% butane. Part of this is to raise or lower the RVP value of gasoline. Summer blends have a lower RVP (Reid vapor pressure) to prevent gasoline from flash boiling or vaporizing too quickly in open containers in high summer temps.
That's the reason for altering the blends from summer to winter.
Now if you look at the BTU values of Butane and Gasoline. Butane has a lower BTU value, so you will have to burn more of it to get the same output of Gasoline. Thus adding more butane to gasoline will have a negative effect on it's energy output. So you will need more winter fuel to get the same output as summer fuel.