Replace
economy in Vel's first sentence with
efficiency and you got it.

(me <-- smartass) ;-)
As far as I can tell, efficiency is good for three things:
1) minimizing the loss when lab and economic allocation are too far apart
2) allowing you to build more bases without causing additional drones
3) loosing less energy due to inefficiency when your bases are far apart from the HQ.
1) is completely eliminated when eff is +4. The other values are very good with +4, but if they are higher than +4, it's even better(especially for big factions with lots of bases, obviously).
As far as I can tell, the interrogation is just a flavor for saying that the faction is eliminated by you and has not restarted. Hearing the scream of your enemy is just a nice reward for long hours of leading a war.

In ACX with the patches, it also means that you keep him safe and that another faction can free him with probe teams.
Another term that's not explained in the manual, but is incredibly important is population boom (+6 growth): It means that, if a base has at least 2 extra nutrients, it will grow by one size in that turn, regardless if its food tanks are filled up or not.
-joer.