Students who emotionally felt personally in control of their lives did better on the classroom examination. Unemotional, intellectual belief in personal control seems less personally helpful in certain situations. However, this research is very new and primitive. We need better measures, better understanding of what is happening, more insight into beliefs in self-control and placebos, etc. Perhaps the instructions to the self-raters in c encourages more unbridled optimism and pessimism, which leads to more variable scores and accounts for the higher correlations with performance.
Coaches everywhere seem to think so.
Some psychologists believe that excessive self-confidence could cause problems, not just in terms of appearing arrogant but perhaps by causing failure since you don't see your limitations and may, thus, overextend yourself. These consequences seem likely but there is only a little evidence, thus far. In short, a combination of realism and confidence seems to work best. People who believe they are efficacious tend to see their successes as resulting from high ability and their failures as resulting from a lack of effort. As mentioned above, an over-estimation of your ability might encourage you to test your limits and maximize the effects of positive expectations.