Just a thought:
1. When a terror mission raid begins, generate a random value of, say, 30 citizens (+ random modifier according to map size).
Drop one for every 1 hour that passes between the notification of the terror raid, and the arrival of the dropship. At the end of the mission, reimburse one citizen for each hour the dropship was in the air (after all, it's not like the grunts can teleport to the mission site so in-flight deaths shouldn't be held against them).
So you get something like this after the mission
N civilians saved
X civilians killed by aliens
Y civilians killed by friendly fire
Z civilians killed before we responded
Success of mission = N / (X*Y*Z) * (modifier based on nation's happiness rating)
It wouldn't involve AI changes but would incentivise you to respond as fast as possible to terror missions.