Is it some built-in safe restraining?
Because when I hit 4000rpm it is like a flick of a switch and it realeases a lot of more hp by itself, without changing the pedal (floored).
Is this correct or is it some problem with the car?

Question 2:
Power steering is really heavy in slow speeds.
Oil and so is of course checked.
I even tried to override the plunger in the power steering by supplying 12V directly. And then it was easy to steer (at low speeds....).
I believe this is a common problem but do anyone know if it is easy to fix?

the 4000 rpm cut off is the rev limiter.
hard to steer is possible plunger valve or control unit. Mercedes had a bulletin to replace control unit so i would get a scan tool and check for fault codes in the traction computer.