OK... let's see if I can wrap my brain around all of the info you provided.
The Samsung Exynos Octa-Core CPU really is nothing but a 4 + 4 design; a quad-core ARM Cortex-A7 Class CPU and a quad-core ARM Cortex-A15 Class CPU smashed together onto the same silicon.
Whereas the Snapdragon 600 CPU that's going to be featured in the US version of the Samsung Galaxy S4 utilizes a Krait 300 CPU which is an ARM Cortex-A15 Class CPU. Right?
So if I'm right so far, then tell me this. Since the ARM Cortex-A7 Class CPU is supposed to be a power-saving CPU that's used a majority of the time and the ARM Cortex-A15 Class CPU is only supposed to be used when the uber CPU power is needed, how is this going to effect battery life in the US version when the US version only has the higher-end ARM Cortex-A15 Class CPU?