The biggest benefit of v15 is that it has full FlashLite3 support (Which means you can view vlash video in the web browser - E.g. For Youtube etc.). You can read about the firmware
here
Debranding isn't too big of a job, you can find the instructions to do se,
here
GPS will only incur charges if you download the maps you are trying to view, while on the go. Use
Nokia Map Loader to pre-install the maps. There are small charges when using A-GPS, but this is minimal