so finally a follow up.
clutch plates were gone indeed.
overheated and the burn marks and some glazing are clearly visible
paid a good deal of money for it, mostly parts cost (total about 480 euro). labour was not so expensive. Even though I read up a lot on the process, I was not able to do it myself since I have to ride daily on her. I might try it next time though.
All in all an expensive afternoon
I guess a couple of track days, all the engine braking and that specific sunday is not a bad deal on the clutch plates, they do wear a lot faster than on a car (about 4 times is what I've heard)
width of the friction plates over the friction material was about 2.85 mm to 2.9 mm (differed on some plates). The specs in the hanes manual specify that new is 3 mm and mininum width is 2.8 mm.
1 mm over about 8 friction plates (forgot the number) is about 8mm of extra space in the clutch, so you can imagine that this is hard on the springs and makes slippage of the friction plates and other plates more noticeable