sfdon, that's an interesting note from the Subaru forum. My college econ instructor would always preface his remarks with the expression "ceteris paribus" meaning "other things being equal". Assume for the moment that Subie airflow is much the same as under the hood of a CS... they both have radiators, engines, exhausts... but a blown and intercooled Subie surely has a lot hotter exhaust than a CS. Nevermind that, we're considering whether an unwanted high pressure area exists.
My 2002s and CS will/would run to about 50-60 mph before an unlatched hood started to lift, so there is unbalanced pressure. I are engineer, so: say the hood area is 4'x4' or 16 sf = 2300 square inches. A hood that weighs (say) 20 pounds could be lifted by 0.009 psi, ignoring downward pressure on the hood from airflow above. Actually I think the hood is bigger and unless the latches are sticking, needs less that 20 pounds to lift the back edge, implying that the underhood pressure is even less at those speeds.
Data point... NASCAR and many high-performance cars (I was once under a Ferrari 360) have flat undertrays. This alone suggests there is something to a undertray, although those cars are intended to operate far beyond the performance envelope of most 3.0CS.
... "more experimentation is needed" by someone with a manometer that could compare air pressure at various exterior locations to the underhood space. I'd guess some thin aluminum could be folded/formed and screwed on at various places. IFF it was "properly" shaped and vented (louvers? NACA ducts?) it MIGHT depressurize the engine compartment, leading to increased airflow through the radiator, POSSIBLY leading to lower coolant temperature. Based on the Subie forum comments, the effect might be higher temps at some speeds.
This being the real world, things are never equal, and my inclination is that the undertray is of very limited utility. Too, it will catch any tool you drop, preventing easy recovery of said tool from where it should fall: the floor. DAMHIK.
For my 2 cents, the undertray should be made from something oil-resistant and resilient, e.g. polyethylene, ABS, etc. Metal, CF, fiberglass not sufficiently flexible.
