Here we have the diffuser from the Ferrari at Hock. A couple of things strike me. One, look at how close to the wheel the diffuser wraps. Ferrari also run a fence on the rearmost bodywork forward of the rear wheels. They seem to be trying to isolate the turbulent air generated by the rotating wheel from interacting with the undercar flow.
Also look how the diffuser and the lower wing element are practically touching. With the amount of interaction between the rear wing and the diffuser, they could be described as two parts of a single aero device rather than two seperate devices.
The inset shows the diffuser prior to Hockenheim.