Because Nissan did a DAMN fine job tuning the suspension, and because it's engine is mounted behind the front axle.
All wheel drive doesn't really help handling at the limit like most people think. What it does let you do is get on the gas coming out of a turn faster.
The only problems the weight of the Z adds to handling is that it does have a tendency to want to understeer. But to encounter this understeer, you have to push it REALLY hard. It's so far out at the limit that it's usually never a problem (and if you don't know what to do to correct it while driving THAT hard, you shouldn't be driving the car!) And frankly, the WRX (don't know about the STi) understeers FAR worse than the Z. Ok, I know somebody here's going to say that with a new shock/spring/swaybar combo, it eliminates understeer on the WRX. Well, same goes for the Z. I do expect the STi to be much better, but I'm hear to tell you that it'll understeer before it oversteers just because it's being sold in the American domestic market. That's the way it works. Sorry boys and girls!
To be honest, I never notice the weight while driving the Z. Granted it's no Miata, but the suspension is so well tuned, and the steering gives such good feedback, that weight is never a problem. Like I said before, performance-wise the STi and 350Z will be neck-and-neck, at least around a road course. (If you're drag racing these cars, you're completely in the wrong parking lot!) The only difference will be your preference for...
1. NA Rear-wheel drive looker, or...
2. Turbo All-wheel drive practical.
Whichever you choose, I think you'll be happy!
