I hope you do realise that for an increase in to speed from 320 kmph to 400 kmph, we are talking about the total K.E. of the car increasing by around 60%. Thus a 60% extra energy will be dissipated in a crash. Now, i am not a genius in track safety, but i believe if Rockenfeller's crash in 2011 had dissipated that much more energy then we would not be seeing him in the paddock this coming weekend.
The notion that drivers and organisers these days are scared to see the cars go fast needs some rethinking, to say the least. Racing is dangerous, period. And all involved, have always known it. People die in crashes much slower than the 400 kmph we speak about here. What the governing bodies do is just give the racers a track that they can push hard on and still have a margin of safety enough for them to return home with or with out their car.
So in short, there is nothing wrong with having the 6 km straight, till something goes horribly wrong, that is.