Just about all production cars generate lift at speed. The 911's shape is kind of like a wing, but I don't know that the 911 generates significantly more lift than, say, an M3 from the same era.
A front spoiler disturbs the airflow as it goes under the front of the car and can also deflect some air off to the side. The lower down you put your front end, the less air will get underneath the car as you're driving. If you put it far enough down, you'll create a low pressure zone under the car. That said, a factory front spoiler will reduce the amount of lift the car is producing at higher speeds -- but it will never result in the car getting pushed down. It will not generate net downforce. The car will still be generating lift.
The same is true of spoilers in the back, and of most of the 911's factory rear wings. You can mitigate high speed lift, to greater or lesser extents, but you're not generating actual downforce. Pazuzu's car is going to feel more 'planted' relative to a non-tailed-and-spoilered 911 that's generating even more lift, but Pazuzu's car is still generating lift. In other words, it's still putting more pressure down on the tires when it's sitting in the garage than it is at 140 mph (or even 80).
'Ground effects' generally refers to the use of a channeled aerodynamic device on the underside of a race car that compresses air in a zone ahead of the front axle, then directs it through aerodynamically clean channels, and speeds it up as it exits through a diffuser in the rear. The sped-up air produces an effective low-pressure zone and can actually suck the car down with a great deal of effectiveness. But cars that generate significant amounts of downforce with an underbody effects kit require a suspension with almost no travel to it at all. For those kind of effects to work, it's important that the underside of the car be very close to the pavement (about 2.5 inches, usually), and that the whole piece be held in a consistent position relative to that pavement. Braking, accelerating and turning would all upset that positional consistency with the torsion bars and springs commonly available for the 911.
Flattening out the underside of a production car like the 911 can reduce the amount lift it generates. Turbulent air will produce drag, and high-pressure areas under the car will contribute to lift. But that's not the same as ground effects, and it's not going to generate net downforce. Using a deep lip in front can also help, as I mentioned above, in reducing the quantity of air getting under the car.
Using a front splitter to harness the high pressure air ahead of the front bumper can help with actual downforce. But that's not the same thing as a front spoiler.
To generate actual net downforce on a 911, you need a pretty serious front splitter, down low, and also an obnoxiously big wing on the back. No factory aero options for street 911s made before 1989 produce actual net downforce at any speed.
