The calculation is already best/worst case. I simply calculated the enthalpy of 3 liters of the mixture. The energy due to the pressure buildup is negligible. I

The calculation is already best/worst case. I simply calculated the enthalpy of 3 liters of the mixture. The energy due to the pressure buildup is negligible. If you didn't make sure that there is no air in the bottle, then the energy would in fact be considerably lower simply because then the bottle is mostly air (the way to make sure is by submerging and filling the bottle with water and wait until the bottle is completely filled with gas, and then closing the cap under water). BTW, speaking of pressure it's unfortunate that these days the soda bottles are so thin & weak, some can't even stand 5 bar anymore, they used to be a lot thicker and stronger.