I think it's a mistake to assume that the new Fusion-based Taurus is going to be significantly lighter than the current car. An AWD V6 MKZ weighs 4,129 pounds. Stretch it a bit, widen it a bit, throw on a couple of turbos, and some bigger wheels/tires, and its not unrealistic to assume the new Fusion/MKZ will pick up a couple of hundred pounds and weigh exactly what the current SHO weighs. It also won't have the current car's giant trunk. The interior will be slightly roomier, but the current car is very comfortable and roomy, its just not quite as space efficient as the Fusion. The small amount of extra passenger cabin capacity in the Fusion doesn't outweigh the current Taurus' massive trunk capacity for me. I suspect that if most people really took a long, unbiased look at how they actually use their car, they'd come to the same conclusion.
The current car is much maligned in the media (and by people on this forum) for being overweight. It's not. It pretty much weighs the same as any other comparably sized/equipped car. This is an easily verifiable fact (Google is your friend), so I am baffled as to why so many people blindly proclaim the SHO to be overweight.
I'm staying neutral on this until the new Taurus hits the streets. Until then, I'm skeptical that it will be anything more than an incremental improvement over the current car, if that.