Didn't the 993 GT2 racer have that huge spoiler?
That's what I was thinking too. Any Porsche experts here to chime in?
Well, I raced 964 and 993 GT2 and RS Americas back in the day. While the street version had the spoiler you see in speedycatz's post, we always added the extension at the track. This was to get the wing up into the better airflow, making it more effective at keeping the car's arse on the ground at high speeds.
Contrary to cxo's post, Porsche 911's have always had a tendency for lift at the back. This wasn't because of engine placement, but because of the car's sloping rear. Aerodynamically, it's terrible. It doesn't give a clean break, so there is too much turbulence, which creates lift at high speeds. Having the engine in the rear makes it worse when you get to the end of a long straight and you start braking, as you combine the aerodynamic lift with the pendulum effect of a rear-engine car.
On the old torsion-bar 911s, I prefer the clean look of not having a spoiler. And that's fine for the street, as long as you don't drive on the autobahn. But on my own personal 1983 911 SC Targa, I had the front and rear spoilers from a 911 Turbo. Because, um, well, I didn't always obey the speed limits.