Shigehiro Oishi and Minkyung Koo have written the interesting chapter 'Two new questions about happiness - "Is happiness good?" and "Is happier better?" in The Science of Subjective Well-Being. In it they explore the possibility that the effect of happiness is curvilinear. They discuss several studies which suggest that the optimal level of happiness may not always be the highest level. These studies showed that on several performance criteria the highest performing indivuals were not the most happy ones but people just below that level of happiness.
Also read: How happy is happy enough?
Also read: The optimum level of well-being