[aerohead]

The more energetic the airstream is under there, the more radical the diffuser contour can be, up to some threshold.
As Tesla demonstrates some of the best underbodies in the industry, it follows that they'd have more latitude on design.
The plaid is high performance, and I presume that high-speed stability weighs heavier on the diffuser shape.
The 'slower' P85D Model S has a much more gentle and relaxed diffuser.