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mcrews,
From the top photo only- it appears the exhaust system is recessed and unlikely to receive airflow from directly underneath. In the front there appear to be vanes which seem to be directing airflow toward the center- but also away from the wheel wells. Also, is there ducting (?) in the front wells (inside, behind the axles) into the exhaust system channel?
The question is "is the channel simply a radiant barrier, does the ducted air to cool the car above the channel, or is it ducted air meant to cool the system".
Since I don't see any "fins" to increase surface area of either the channel or the cat/muffler, I'd say the Audi engineers weren't betting on big chances of conductive losses. Also, and article I just read doesn't suggest any great temperature differences whether at idle or at speed for the warmed up auto. Since conductive losses would improve with the mass of the air that came in contact- I would assume conductive losses to be considered negligible by engineers.
So once again I'm just guessing but I'll throw out some numbers now and make my bet: Mechanism for temps <300ºC = convection
Mechanism for temp >300ºC = radiation
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