I have a Rossi Pixi Gold head.
With the standard plug that comes installed, I think you have to use at least 30% to get any kind of top end.
The optional turbo button that it comes with, installed with a p8 plug, the engine really shines.
I only recently found this out. I'm a little new to italian engines.
With 20% Nitro the engine won't ever top out with the standard plug installed and shimmed buttonhead.