Not to scientific but both were using same build and fully charged battery
Standard mode before update: (notice this one isn't fully depleted, but I got maybe a one more pull before it said low battery)
View attachment 459774
After update using eco mode: (this one was reading low battery if I tried to fire it)
View attachment 459776
Its not magic, in fact, there is nothing special with Eco mode accept its turning the screen off after 5 seconds instead of a full minute, yes, its also been reported that it limits your Joules output to 40 or under but that is likely not going to be many, the bigger factor is the screen timeout.
Look at your smart phone, the screen is likely the biggest contributing factor to battery drain, this is no different.
Instead of Eco mode, they should have a screen timeout time for ALL modes, want your screen to turn off but still be able to use Soft, Powerful, or Powerfu+? Oh well, too bad, you cant (unless you want to hit the Up button every time you vape, which isnt an option).