Update: Disassembled the clutch yesterday and took all measurements which are important. I will post them here as they are quite interesting.
Lets start with the old (in fact new) kevlar clutch plates I installed a few weeks ago.
Clutch friction plate thickness (in mm): between 2.72 and 2.74mm. Only one plate was 2.72 most others had 2.73 or 2.74. All friction plates together showed a height of 22.22mm. All steel plates had exactly 1.5mm so the stack of steels was 10.5mm. Total height of steels and friction discs: 32.72mm
Width of the teeth of the friction discs was between 13.74 and 13.82mm Only one tooth was over 13.8mm, most were between 13.75 and 13.78mm
The distance between the "fingers" (dunno the English term) of the Rekluse basket was between 14.00 and 14.06mm. Considering the difference between the 14.00 of the basket and the width of the teeth of the friction plate there was enough room so that none of the plates had a chance to get tilted and therefore blocks the clutch.
Clutch pressure plate lift:
with OEM slave cylinder: 1.22mm
with new OEM cylinder: 1.33mm
Difference in clutch plate lift with clutch lever in its farthest position and its closest position from/to the bars: 0.3mm (1.22 vs 0.92) This I tested only with the old slave the bike came with. When lever is closest, drag increases significantly.
Just as info: the new style slave has the same cylinder, at least visually it seems to be identical. The new piston is slightly higher and carries two o-rings that seal against the piston. The internal spring is very soft.
The slave the bike came from factory has a lower piston with only one o-ring and a much, at least 5 times stronger internal spring.
Feel on the lever with the new slave is somewhat different, feels a bit spongy towards the end, before the lever touches the bars. Also the pull is slightly firmer.
The new friction discs looked different but exactly as the ones I took out a few weeks ago and unfortunately binned. On the pictures below you can see that it says "Adige" on the plates. That is very likely the producer. You can also see that these plates teeth went through a face milling process.
Thickness of brand new friction plates (dry) between 2.71 (thinnest and only one) and 2.75 (thickest and also only one). Most had between2.72 and 2.74mm.
Height of all friction plates: 22.19mm, so just a difference of -0.03mm. As I used the same perfectly straight steels the height of the whole package was also 0.03mm lower, exactly 32.69mm.
I tend to say the tolerance of the caliper rule is somewhere near 0.2 or 0.3 so there is no significant difference in the stack height.
Also the width of the teeth of the new friction plates was exactly 13.74mm.
Here two pictures: first the old clutch plates which were in fact only about 20 hours in use. Second one shows the brand new OEM plates from GasGas:
After we made all the measurements and a thorough visual inspection as far it was possible without taking the hub and basket out I was really sad as from a logical stand point the replacement of the plates would not have changed anything. So I decided to put the old plates in again.
My friend then convinced me to give the new plates a try and I really did not want to put them in but finally agreed.
Startet the bike and put it in gear and it slightly dragged forward and dragged even more when I started it in gear. But it always did that when cold and it was significantly better than with the old friction plates. Then let the bike idle for a minute and made a test drive. Clutch action was fine and I had the impression that it shifted smoother.
Then stopped the engine and waited 30 seconds, re-started and put it in gear and: nothing, no jump forward no noises. Rode it for a minute, stopped engine in gear, waited 25 seconds re-started, only minimal drag. Almost perfect. Tried this about 30 times, then really let the engine heat up but all was good.
So why I am not happy (yet) ? Well, because it was as good when I first replaced the friction discs and the problem appeared after a first ride in the enduro terrain. It was good for 3 hours. The next ride it was there. It didnt start to get worse, no it was there after the first 15 minutes.
Unfortunately I will not be able to ride before first week September but will inform you when I am back. I hope it won't squeal as my next ride will be in tough terrain but in the meantime I wished I had not started to get rid of the squeal which was by far less annoying than the dragging issue.
In the meantime, many thanks for taking the time to read all this and for all your help and very useful hints. Much obliged, guys!