Thanks for this thread and everyone's inputs, I've learned that the US firmware did shorten my P600 battery life to a maximum of 6hrs screen time, with GPS off and power saving on. After flashing the UK firmware stock, it now reaches 9hrs of screen time with heavy use (mostly youtube & web surfing). The standby drop in battery is also significant less, and it should get better when the software sets in as some users reported. Right now I'm happy with the tablet with GPS and Wifi turned on 24/7.
The best part is when I flash UK stock fw via Odin 3.09 and didn't do a factory reset later, every personal apps, data and settings are kept intact so I didn't have to set my tablet up again. Some would suggest a factory reset right after flashing stock fw to be sure, but I'm happy where I'm at now, I won't ask for more. Hope you all will find the same or better experience.