It isn't like nothing has happened, but I've been busy at work and didn't really feel like doing much at the computer.
The S90 had some more testsessions, swapping exhausts with different bikes.
Conclusion: the camshaft has a timing for an open megaphone.
So the cam was swapped for another, still a hotcam compared to a standard S90 cam, but not the wildest one.
After the cam was swapped, we did another test, it started easier, and responded pretty well to the throttle, with the intended exhaust with muffler.
Also much quieter, what means power to the rear wheel instead of making noise.