Yeah I think you are right, they would provide conservative figures,
partly I guess due to the super car scare here in Australia of the early 1970's and because they provide a safe benchmark, if you drove the 6.9 on the German Autobahns at 200-220 you wouldn't expect the motor to be revving past its limits and at risk of breaking anything.
What a sight a 6.9 must have been on a stretch of German Autobahn in 1979, when most cars could barely cruise at half the speed.
These days BMW and Mercedes have a gentleman's agreement with the German Government, they won't impose blanket restrictions on the autobahns, even though a growing % of the roads are limited in exchange for the car makers will electronically limit their cars to 155MPH, thats only a tiny bit more than what a 6.9 will do.
Just think for a moment, you are in 1979, sitting in your VW Beetle, on the Autobahn just outside Munich, say cruising at 90-100k's, some German battleship r6.9 oars up beside you in doing 200k's and fly's past like you are stopped.