From what I've heard from other people that have the subi navigation, it doesn't sound like it is worth it to me. I have heard that it is less than steller. I have not personaly used it, just going off what I have been told. The trade off is you will get a more flush look but if you will be upgrading the audio system any way you will probably be getting a new head unit I'm guessing so you would have to pull out the nav system any way.
On another note, most things rarely stay current after 10 year in terms of technology.