Update:
Lucky me found a 40 pilot jet, original Keihin in my toolbox
So this morning we took the head off, as expected that was a no brainer. Pretty easy job as on most two strokes.
We measured squish and it was 2.05mm on the stock head. Then we did the same with the GP head (which surprisingly looked exactly like the stock head and was not black like on the GP bikes delivered ex factory).
With the GasGas GP head we measured 1.79mm. The difference of 0.26mm seemed not be very much but in fact that makes a huge difference. I could feel that the kickstarter was harder to move (by hand) than before.
First I wanted to try the head and then change the pilot jet but then decided to change the pilot jet and make test run and carb setting later. I left the NECW at #3 and did not change anything, then started the bike. We had 23 C which is about 74 F but in the sun it felt more like 85 F.
First thing I immediately noticed was that the idle was far to high but I left it as it was as I thought it may come down with the engine getting warmer. But it didn't. So I turned the idle screw half a turn ccw and the idle was nice and stable. I have to mention that I always set the idle very low. I prefer a bike that stalls from time to time than a too high idle.
My riding buddy immediately said that the bike smokes less when I blip the throttle. It used to smoke after each blip but that is almost totally gone now. Then I rode it up and down the road with medium revs and it felt very crisp. After a while I gave it a hand full in second and third and I was very amused as it is so crisp and responsive. It never felt that good. I tweaked the air screw a few times and found the sweet spot at a tad more than 1.5 turns out.
After a few minutes I had to stop as one of the neighbours called the cops and told them there is a bike without licence plates going up and down the road with a fat bastard that wears shorts and no helmet.
When the coppers arrived and asked what was going on we said we haven't seen anyone and it was maybe in a byway about 300 feet from here. When they moved I did a last short run, just for the idiot who called the cops and I whacked the throttle full open in second and third and the upper triple clamp tried to hit me in the face twice
Niiiiiicccccee!
Buddy said exhaust smokes a lot less than before and sounds nice and deep, just a tad lighter than before, but no indication of being too lean.
The silencer was completely clean. Before the head change it had little oil spots from just one acceleration through the gears. This time it was dry. So it seems spooge is reduced too. The best part is that the engine doesn't sound lean or bangs on decceleration and it feels so super crisp.
Of course I can't say was it the head or the pilot jet but I guess both things helped the engine to perform as expected. A lot better than before.
Final verdict has to be postponed until I can ride it on the usual play ground but this may take another two weeks. So far I am pretty impressed as it never before run that good.
Thanks to all for the useful hints
edit:
current setting:
Main jet: 172
Pilot jet: 40
Needle: NECW
Clip: #3
Air screw: exactly between 1.5 and 1.75 turns out
Slide: 7 without notch
Temperature: 74F
Altitude: 650 feet
Oil: Motul 710 @ 50:1
Fuel: 95 octane pump gas