Well, seems like the mounts are not the reason for my problem...
Checked them today, no leaks, when you lift the engine, they work like expected.
But being in the garage, I had the chance to rev the engine up in idle for some time.
And guess what: the vibration is also there when you rev' it up without a gear. You can't hear but feel it.
So summing up the problem it seeks to be like this:
Whenever the engine does more than roughly 3000rpm, there is a vibration. The vibration is changing its "tone" as if it is pulsing. It is not a steady vibration and sound, it is pulsating.
When the car is running you can feel and hear it more easily than in idle.
With increasing rpm, the vibration gets stronger, and the rythm is increasing too.
Above roughly 5000rpm the entire car is shaking.
Loosening an engine mount seems to intensify the vibration. So does the installation of the venom brace.
The problem occurred the first time, when I installed the brace for the first time. Anyhow, I don't think the brace caused the problem but may have been the straw that breaks the camel's back...
The garage suggested to install a new flywheel.
Which sums up at about 2.500 Dollar here... To be honest, I have no idea what to do or check...