Hartmut, actually the suspension pretty much stops working at about 150, you are traveling about 220 fps, how fast acting do you think the suspension is... say you run over a low spot that is 2 feet across....
Now without a doubt the first mile will be a little rougher. The rigid rear is not set in stone 
just one of Max's thoughts to make things a little easier.
Now going 300 will be your task... 
I will start here by listing my engineering qualifications followed by my extensive experience in designing suspension systems.
Ok, now we've got that out of the way I will relate why we chose to go with a "hard" rear end on our lakester.
1./ I saw that many succesful cars lacked suspension.
2./ I saw that many teams reduced the travel of their suspension and stiffened it progressivly as they tried to control stability.
3./ I failed to find anyone with convincing evidence that there were suspension systems capable of tracking a surface at 200+mph .
4./ Throughout the build of our car I clung to my own dictum that "no science is better than bad science" , the only thing I ever heard about suspension was "some is better than none"......Personally I think the advantage of the simplicity, the clean aero and the safety aspects when a car is sliding are very great benefits indeed.
In the first year we ran our car, it was incredibly smooth, and sat on the track so well it was boring. It turns out the trouble I had this year with vision and handling are most likely down to a severe balance problem we had which ultimately destroyed the motor.
I am happy with what we built and dont regret not using any suspension. If I could condense the argument , and my opinion, it is based on my suspicion that I don't believe anything could have a response rate , spring rate and a sensitivity to keep the wheels on the track any better than nothing other than the tyres and the damping provided by the mass of the body.
Other's results may vary..............
