Especially at the rear deck lid extension they used. I think that would help you a lot, considering how steep your rear window is. You probably have the same aerodynamics of a full wagon or hatchback, completely ignoring the trunk.
We ran out of CO2 from MIG-welder, so we were not able to make diffuser, but still we were able to make small extensions to rear wheels.
Belly pan was also finished. I decided to end it up on point of oil drain plug. This way I can be sure that engine-/gearbox oil will cool down enough, also keeps the maintenance simple. Lets see if later on we extend it a bit.