Depends what you mean by insane. Aleister Crowley wrote some of the finest works on the occult. But to the general public he was insane.Insanity has many forms, just as other mental diseases have. And that's what insanity is, mental illness. Megalomania (Adolf Hitler?) is a form of insanity.
Now, the question is would insanity affect your "abilities". It would probably affect some, sometimes all. But some of the great composers,great scientists, great explorers were all quite bonkers!