I posted something on another forum about maybe the front diffuser's job is to minimise the air going into the wheel well. It could do this by having its forward facing section narrower than the rear one (as per the pic in S. McBeath). In this case both the high pressure in front of the wheel, and the higher pressure in the channel itself would, maybe, create a wall for the faster moving air on either side to be diverted under the car where we want it..
I disagree entirely. The front diffuser is all about downforce and aerodynamic balance. You can tell they are aimed at downforce because the front of the car is designed to feed air to the diffuser. If you ran a super low splitter in front of a diffuser it would stall and not produce downforce, thus it is ok to feed a little bit of air under the car for downforce generation. This technique probably decreases the size of the stagnation point at the front of the car with the associated drag decrease aswell.
One influential factor in sportscars are the strict rule limitations that are continually being introduced on underbody stuff like diffuser length and width, flat floors (with no skirts). There are two effects:
a) Smaller rear diffusers require less air
b) Need efficient way to gather front downforce with low drag
Proper front diffusers have only been around since late 90s - before that they had front mounted wings (see Jaguar XJRs etc). But they produced tons of drag for all their downforce. Not very efficient L:D ratio.
The rules are often more influential than you'd think and interfere with the perfect aerodynamic design. On the V8SC, the cars are so heavily restricted I'm sure there would be some rule that says you can't modify the factory bodywork above the sill line. Again, they have practically no rear diffuser and shithouse control front tyres so anything gained at the front end is absolutely crucial. This is probably slightly less true for the Aston GT1 car as they have a huge rear diffuser and a little less rule limitation.
I can't speak for the DTM rules governing the Audi photo above, but it is still a cool example
I wish I'd taken a photo of the actual underbody
One thing you could do is pull the wheel arches tight in around the wheel and extract all the air from the top. I've got a photo of an Audi R10 wheel arch I can show you when I get home. The leading edge is sharp whereas the trailing edge is radiused.
Lurchness, I think you should read up on Bernoulli's principle particularly the conservation of mass part
Air under the car is always low pressure (when the car is moving)!