Sorry to contradict you, but nowhere would a Porsche 956 (or a 962 for that matter) be faster than a Sauber C9. This Mercedes was the fastest car on the 89 Sportscar Championship, beating cars like the Jaguar XJR-9 and XJR-12, Porsche 962C, Nissan R89C, etecetera...
To have a car going really low 5 minute laps on real life would be almost impossible, even for a current F1, as the car would get airborne a lot of times if going full throatle (let me see: Flugpatz, Schwedenkreuz, Fuchsrorhe, Kesselchen, Hohe Acht and the at Plantzgarten Eins und Zwei)...