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I'm beginning the question the whole interference drag issue simply because the underbody doesn't act as a traditional duct. There may be interference drag between the sides and underbody (which always exists) that can be mitigated with radiused corners, but there should be little in the way of underside and road interference.
On a calm day, the road surface is essentially moving at freestream velocity, therefore the boundary layer can't be thicker than the ride height. The lower your ride height is, the thinner the boundary layer. If the road was static, then I could understand that interference might exist...but that's not reality.
Compression certainly exists, but as Trebuchet03 noted, it is not significant. Air isn't considered "compressed," even though it is, until it's density has increased ~5%. Incompressible objects, such as water and steel, are compressible. It's just that any change in density is extremely small.
- LostCause
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