If one were to upgrade to a 14" BBK on a 330, would they need to get the rears as well? That is, do the rears contribute a significant amount, that you would the feel the difference on the track? Is it worth it to save up for the entire set, or just get the fronts and either stay stock, or get some Cross drilled rotors for the rear as well.

Depends on what your purpose of use is. If you are running DEs and autox front upgrade should be sufficient.