30% is fine for that engine. Overheating is usually caused by running too lean, but running with RPMs too high, or not having enough air flow over the head can do it too. I run mine on 30% cool power, with an Enya #3 (hot) glow plug in the winter months, and an Enya #4 (medium hot) glow plug during the hotter months. I have never had an issue with my engines, other than normal maintenance.
Are you 100% sure that you are not running it too lean? Needle settings are going to vary according to things like elevation, humidity, ambient temperature. So you can`t really rely on the needle settings suggestions in the manual. Try richening it up a bit, and then work back into you tune. If you`re not doing hard 3D, you`re better off running it a little on the rich side anyway. The engine will last longer.
