I'm using the same car as yours, and installed 2 amps, front comp, 12" sub and soundproofing on front 2 doors. If you do really want to get the most out of your speaker, then the soundproofing is a must cause our jazz is like tin milo actually.

As for the concerns about warranty, it only void the sound systems area, which I have been told by the SA and SC. One thing good about our Jazz is that they got RCA out so no wire cutting is needed on the originals.