This thread covers it fairly well.
http://ecomodder.com/forum/showthrea...oss-13556.html
If you add weight, your car will have more kinetic energy at any given speed. This means longer glides, but equally longer pulses. If that shifts you into a regime where you can P&G better, as it did for Brucey, then it's a good thing overall.
For most people, the increase in rolling resistance, and the dramatically increased amount of energy wasted to braking (when your glide goes longer than you'd like) mean that adding weight is bad for FE.