Dear, I am no authority here but if one is confident in his expertise in a particular subject than one has to opt that subject, irrespective of the fact that its scoring or not.
Topper CSS-2008 had Urdu as her optional subject. She said in her interview that she was avid reader of Urdu lit. She managed to score more than 120 in Urdu which explicitly says that you can score good if you have in-depth knowledge of a subject even if that subject is not scoring.

Regards,

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