Because there is no power to weight ratio for RS6, and that negates RS6's points. That's why the curb weights should always be added.
You should update the data for both RS6 and RS7. There were lots of drag tests already (see DeDe's / benzsl's comments on both pages.)
I would take a RS7 over RS6, because I prefer 4-door coupe's exterior more. In anything other they are absolutely identical.