The road test techniques varied considerably. C&D beats the living heck out of their cars. R&T used to use two testers in the car, though new test equipment in the late 70's eliminated the second person to operate the stop watches. R&T tried to shift quickly, but the drove the car like the were paying for it and planned to keep the car for a while.
You will note that in every case, the 80-83 US versions of the SC are slightly less quick than the 78-79 versions, even though Porsche claimed the same power ratings.
Regards,
Jerry Kroeger