My guess: Generally it should not be that different since most of it is handled by the EC firmware. (Proprietary is OK in FSF’s eyes if it’s baked into the hardware) But Canoeboot does not include microcode updates which could affect the CPU’s power management. (Clocks speed, Vcore, etc.)
You are confused and mean. You cannot compare 2 different laptops and say that one has worse battery management because of coreboot. You would need to have the same laptop with and without coreboot to do any comparison.
I don’t really get what you are trying to say in the second paragraph but Canoeboot is not a good daily driver because it’s basically a “drug substitute” for GNU dummies. The author of it does not recommend using it. Use Libreboot instead as it does not break your CPU.