Here is my 2 cents about the louvres:
The Top surface of the wing is undoubtedly high pressure. The underside of the wing is lower pressure(I'm not sure on absolute figures but it is a lot closer to atmospheric pressure than the top surface). And outside the end plate is atmospheric pressure.
Without the louvres the large flow rate and pressure differentials cause large trailing edge vortices (the things you see in the wet). The louvre simpley allows a more 'gentle' equilisation at the edge of the wing. This reduces the overall downforce but drastically increases the efficiency, reducing drag.