Thanks guys.
Many said NGK iridium is indeed very good; long life, smoother run, better acceleration, lean engine, but...at high temp d firing performance dropped.
Next question is, can it be the reason that when throttle setting is perfect, after a few hours ride the setting is down and a bit and the graphic at vacum tester became irregular? Wonder what's the connection between iridium plug and throttle setting.
Does anyone know what the gap range is for NGK CR9EIX on Busa GSX 1300R 2000? Couldn't find it anywhere. Manufacturers advice not to gap iridium plugs. I understand it can be done, but i don't know the maximum tolerated range.