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I hadn't seen this thread before, and I note it started 6 years ago. I am not going to enter into a debate about theory, but simply say two things.
1. When you can measure the surface pressures on your own car, at any location, at any speed, whenever you feel like it, a lot of this discussion starts to look very academic. You don't need to theorise or guess - just measure what is actually happening. It is only surface body pressures, and the direction in which the resulting forces act, that determine thrust/drag, and lift/downforce. Nothing else.
2. Pressure distribution on cars depends on shape. Yes, you can generalise (ie use rules of thumb) or look at published pressure distributions for other cars, but literally every car is different. The pressure distribution will depend - among other factors - on whether there is attached or separated flow, the thickness of the boundary layer, and the positive/negative camber of the body's individual parts. And all that applies equally underneath the car. Because of the large panel areas involved, even tiny changes in pressures can have a big impact on the resulting aerodynamic forces. That makes generalisations often quite deceptive.
TL;DR: just measure them.
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