[aerohead]

lunarhighway, your question is a good one.What wikipedia might have mentioned in their article about diffusers,is that they do not work if flow ahead of the diffuser is not smooth and well attached.That usually means a belly-pan.As a palliative to under-body turbulence,front air-dams and rocker-panel extensions structure flow outside the region containing undercarriage protruberances, which would tear and trip otherwise "clean" flow.The "dams" determine where the air can go and endeavor to get the air to just barely clear underbody projections and wheel areas,allowing it to pass in a straight line to the vehicle rear.With respect to the built-up,or multi-panel leading-edge tire fairings,in theory,there's no reason why they wouldn't provide some minor benefit.Their leading edges would require enough radius so as to allow flow separation,and each trailing panel would also require proper radius.Without a central spine,the panels would only work at zero-yaw angles,as in any crosswind condition, there would be no inter-connecting structure to contain the flow and prevent bleed-out.The panels would still be required to respect the same ground-clearance limitations of a unitary fairing.