Im not familiar with your specific deck, and Im too lazy too look up its specs, but your external EQ is only overkill if your deck already has a nice built-in EQ. Otherwise, its crazy to say an EQ is overkill in a car audio system. Even with a nearly flawless install, sound deadening, and top quality equipment, you will never be able to eliminate the sound wave interferences from things like reflections off glass, absorption from soft fabrics and seat cushions, different path lengths, etc. Lets face it, for sound reproduction, a car interior sucks ass. You can improve some/a lot of it with sound deadener, kick panels, etc... but you will never remove all the issues the environment creates. For this reason, a quality EQ will (almost) always be a benefit to a car stereo system. That is assuming, of course, that it is installed and adjusted properly.
Add to that the fact that most people like to tailor the music to their own personal tastes and moods, and an EQ becomes a very important piece of your system. Id question the quality of the experience of any 'veteran' who claims an EQ is overkill.