updATE

When M5 was tested the car had Continental tires and the M6 is using Pirelli tires. The Pirelli tires offer more grip , they are almost like R-tires but the difference is that the Pirellis can be used in the wet. They are more allround than proper R-tires.
So to get the maximum grip out of the tires the asphalt/ the track need to be warmer and unfortuanly when the M6 was tested, the track was cold.
If the M6 was tested during summer or if the road was warmer, they would cut the laptime by several seconds. So a 8 minute laptime isnt impossible with the Pirelli tires during the right conditions. Unfortuanly we will never know if the M6 isnt tested again.

the same goes for CLK-DTM but Sportauto has never tested the CLK-DTM twice so it probably wont happen for M6 either