After alot of testing, this is what has come up;
I've done all of the above, even run NGK 7 plugs now.
The engine run really well above 4000 (up to somewhere... How long can it be stretched?).
It is still hesitating/ grumbling between 2000 and 4000 after braking through a corner and then accelerating. This is because of excess fuel.
Although shifting into 2. gives the punch, I'm convinced that a leaner bottom end will make the car faster as the time used for downshift + upshift will be larger than the time used for accelerate in 3. from 3000 and up...
What will be tried first is:
Use a smaller main fuel jet (from 165 to 145) and a smaller air jet (from 220 to 200). In theory, according to Mr. John Passini (if I read his words correct...?), this will make a leaner low end while keeping the upper end as is. Sounds smart, or?
The 2. choice is to follow Eljaro,
Interesting Weber jetting, and use the F15 emultion tubes ("Tubes with bigger diameter for engines with strong vibration or high compression" and "Leaner low end and throttle response")
Runes