Unfortunately though, leaving the back of the undercarriage uncovered like that kind of ruins a good portion of the aerodynamic benefit of having an undertray. There is a lot of wind turbulence created by the rear bumper cover and all that stuff back there--it's basically a big scoop that's scooping up air from underneath the car.
On every race car I've worked on, adding an undertray that just covered up the rear portion of the car up to even just the rear subframe had a tremendous effect on the car's handling and drag. We did leave the mufflers uncovered though and I suppose that might be a bit more difficult on a Boxster...
