mano42, is it possible for you to try downloading the update via wireless on your Kindle rather than copying the update file over from your computer? And if so have you tried that? That's the only thing I can think of to suggest.
On the topic of battery life: My PW seems to last about the same length of time as my KK. My KK is hardly new though (I got it as soon as they launched in my country and read heavily on it ever since), so I don't know how it would have compared when the device was brand new. They're both 3G models where I keep the wireless switched off when not using it.