Audi used to do a lot of durability testing at Sebring. Not sure if they still do. 12 hours at Sebring is like 36 hours at Le Mans in terms of beating up the chassis and suspension. Very hard on drive trains too. Rough, very bumpy surfaces (concrete and tarmac).
One of the best stories of Audi testing was that of the Audi being on track with an old McLaren M6B (built in 1968). The 725 hp small block Chevy powered 1,400 lb Can-Am veteran shocked the Audi guys with it's huge power and light weight. I believe that it was only Allan McNish who was not surprised by the raw power of the ancient McLaren. McNish had driven several old Can-Am cars at vintage events.
Group 7 in its heyday was simply awe inspiring.