that sort of depends on what the mega structures actually do. if it is some kind of energy source for FTL travel or super weapon or something, it may be worth i

that sort of depends on what the mega structures actually do. if it is some kind of energy source for FTL travel or super weapon or something, it may be worth it to dim your star to get the benefit. also, the mega structures may be built around a star that is not their home system: it could be a trap to get other aliens to send ships to investigate the dimming, at which point the builders of the mega structures capture the investigating ships for whatever reason. dark forest theory and mega structures are compatible under the right circumstances.