Our sense of justice is part of our morality, therefore we should not change it.
I have no premise "if something is part of our morality we shouldn't change it".
"We should seek justice" is tautological. If justice and optimal deterrence are contradictory, then we should not seek optimal deterrence.
No it isn't. See Thomblake's reply. I for one feel no particular attachement to justice over optimal deterrence. In fact, in many situations I actively give the latter precedence. You can keep your 'shoulds' while I go ahead and win my Risk games.
The fact that you do not value something does not serve very well as an argument for why others should stop valuing it. For those of us who do experience a conflict between a desire to deter and a desire to punish fairly, you have not explained why we should prioritize the first goal over the second when trying to reduce this conflict.
Cass Sunstein, David Schkade, and Daniel Kahneman, in a 1999 paper named Do People Want Optimal Deterrence, write:
If we're after optimal deterrence, we should punish potentially harmful actions more if they're hard to detect, or else the expected disutility of the punishment is too small. But apparently this does not accord with people's sense of justice.
Does this mean we should change our sense of justice? And should we apply optimal deterrence theory to informal social rewards and punishments, such as by getting angrier at antisocial behaviors that we learned of by (what the wrongdoer thought was) a freak coincidence?