0.3mm head clearance can be too low when the engine will gain rpm, due play and flex on all internal components the piston can hit the head at high rpm.

If you lower the head you will shift the ignition, it will be sooner. That can cause a lower top rpm so you have to correct that with a colder plug. If you are running a 3 or 4 you should try a 5 or 6.

Raising the sleeve will raise the port timing, a higher exhaust will give more rpm and a higher intake can give more overall power.