It's not that I think diet is unimportant, but I've never noticed any major effects. I've lived in the U.S., Brazil, West Africa, and SE Asia. I always ate the local diet, so my diet has changed a good deal over the years. None of those changes effected my general sense of well-being, and certainly did not seem to effect my piano playing. A constant diet of cassava, plantain, and peanut stew in West Africa did seem to lower my cholesterol, though.
Just my own experience, there are certainly many who feel that sticking to very specific, strict dietary regimes has helped them, but I'm not among them.