Reason for having a tighter gap acording to Oppliger Romain:

Difference between race-engine (like mine) and a road-engine, is the power and pressure created in the combustion chamber.
It make higher compression/préssure cylinder
The spark needs to be stronger inside then a lower compression engine (stock YB or lower).

Anyway, 0.5mm now and pushing strong to 1.9bar.
I'm going to test tomorrow up to 2.2bar, see if 0.5 is still good enough.