+1 also wondering about this. I seem to remember an interview with Sabine, ring-taxi driver, that unofficially the taxi drivers are not allowed to go faster than 8-minute laps, but that she wouldn't acknowledge not having done it. To me, that means the E60 M5 is capable of sub-8 minute laps in the hands of a pro.
You are confusing Bridge to Gantry times (Sabine quote) and full lap times. Sabine and the other professional taxi drivers are not allowed to go faster than 8:30s BTG.
FYI 8:00 BTG equals about 8:30s full lap.
I doubt the new RS6 will be faster than the E60 M5, but it will be close...
best,
adam