Fixed here by limiting the framerate in the nvidia driver for the specific unity.exe. 
At my case 120 fps is enough for the Editor. GPU Utilisation is leveling on idle level.
Enabling vsync also helps but only when in GameView.
Interaction Mode
In Play mode, Unity ignores any throttling that you apply using this preference. The editor updates as fast as possible unless you enable the VSync
option in the, or set Application.targetFrameRate to a value other than -1.
When switching to scene view, while in playmode, vysnc seems to be ovewritten, because GPU utilisation goes up to 100 % again.
After re-enabling the vsync flag while in gameview the fps is limited to the refreshrate of the monitor again. It seems like the scene view is rendering stuff on the gpu even when the scene is empty.
Forcing Vsync in the driver doesnt seem to affect the Scene view.
some more reads