The speed of the decrease of enthusiasm is different for all levels. The further away from 5, the faster your enthusiasm will drop. It's nearly impossible to get an enthusiasm above 12 at gamedays.
It's hard to say whats the best strategy, it depends on your opponents. I tend to play TIE when I either think I'm gonna lose big, or when I think I'm gonna win big. Overall I'm trying to keep my spirit around 9.