To sum it up, yes you get a higher return profit by investing below 100% production.
You have now invested 290 gold in this region, boosting production by 80 % to 168 %. Your advisors hope for a return of ca. 390 gold, if everything works well.
To ensure that your investment pays off, you should try to keep morale and realm control high and prevent enemies from looting. With its boosted production, Sasrhas is sure to be a prime target.
Your continual performances have finally paid off. Your prestige rises one point.
