Well... I decided to keep this device. I made some setup adjustments, turned off some functions I don't use or don't care, plus the magic appears to be made. Today the battery predictions was enabled for the first time on the settings, and the autonomy appears to be acceptable, finally. Not good, but acceptable. I started the usage, as usual, at 6am and now, 10:56am here, I still have 80% of power, with my normal morning usage (newspapers reading, reply to some emails, some videos and messaging), 1 hour screen on. Now it appears it'll last until the end of the day. So, confirmed, 1 week is the deadline to this thing really begin to function well.
Thanks averybody for all the tips.