Like I said before I only used 7 gear one time when I drove the M6. I also tried 7th gear starting at low speed, the car doesnt even notice that you had such high gear at low speed, the car accelerated anyway.
7th gear is good when you want to save fuel and it also depends on gearing if 7 gears is needed.
I found this about the new SMG for the M5/M6.
1 gear: 71,5 km/h
2 gear: 107,4 km/h
3 gear: 157,7 km/h
4 gear: 204,6 km/h
5 gear: 245,8 km/h
6 gear: 284,8 km/h
7 gear: 341,9 km/h