I just went out and took pics of the underbody relative to the bottom of the front bumper, as NeilBlanchard was asking. The oil pan is lower than the front lip, but lower still by a bit are the two exhaust headers, and the thing I use for the rear mounting point of the skidplate.
It's hard to say just how much further down from the lip the car parts are, but I'm going to guesstimate two or three inches lower.
So I'm thinking, an air dam that extends down about 2.5 inches from the bottom of the bottom lip of the bumper, all the way around from one wheel well to the other, and then a partial underbody tray that goes from the bottom edge of that to the skidplate mounting points at the lower-back of the engine.
What does anyone think of that idea?
And I think I discovered some more cause of the road noise I experience.
It looks like the bottom of the car just before the front wheel wells actually
encourages the flow to go up into the wheel well. C'mon, Subaru, yer killin' me! This is not a race car, there doesn't need to be that much airflow into the wheel well!
I'll plan out some front-of-wheel fairings (splitter? spoiler?) to go with the air dam, as well.