True, there are many alternative kernels available and many of them focus on power and battery issues. The OEM of any device has to make tradeoffs between power and performance. They also have to take into consideration that their software has to provide the most compatibility possible for all users.
Alternative kernel developers on the other hand do not have to develop to such stringent standards. Yes, the kernel needs to perform at a certain level or no one would use it, but if there is an app compatibility issue or a slight hardware compatibility issue the answer is always - flash a different kernel. The developer may or may not address the bug in a future release. The OEM does not have that option.
We did see an increase in battery life when Froyo was released, and that would have come with a new kernel. However, to see a kernel specific tweak for battery, it’s not likely.
On the bright side… Root is easily available and the world of alternative ROM’s and kernels are readily available! Unlike the old days when what shipped is what you got and if it was bad… then so sad!