Efficiency is a red herring. Ethics ends up just two sides saying "I'm right you're wrong". The only meaningful question is "Is the government fulfilling its role as government?". Or the simpler proxy question: "Is this constitutional?".
Ethics talk is opinion because obviously the people doing it and their supporters think it's ethical. If you tell them you think it's unethical they'll disagree and discount the rest of what you say. Two groups just saying "I'm right and you're wrong" or "I'm ethical and you're unethical" ends up with the more powerful one getting what they want.
Even if, say, torture efficiently got information, if it also galvanized the world against you, provoked many suicide bombers, got your own people tortured, lowered the population's trust and faith in the government, distanced your allies, increased the costs of maintaining the military, and so on, it might not be worth it.
If you put the entire population in jail, you will have 100% efficiency in jailing criminals. But what cost? The point is that if the government doesn't protect freedom or represent the will of the people then it will lose popular support and have to support itself by convincing people lack of freedom is preferable to freedom or just lying.
Besides, it didn't stop the Boston Marathon Bombing.
This attitude is very dangerous as the government will just change its role.
Everything is rooted in ethics and thats why we should be worried whether our laws and actions are ethical. Just because people disagree on it does not mean we should stop trying to find out objective ethical laws and principles.