Physics is indeed a high scoring subject. Not only Physics but other science subjects as well. But if you are opting for highly competitive Foreign service then IR or Political Science should be your choice along with Physics. The reason being that every group has some subjects related to it, for example Public Administration is more into Police Department. Wise choice of Subject let the Interview Panel and the Psychologist Panel Know that you are not into marks but you want to be perfect for the group you selected. Automatically according to Human Behavior their attitude will favor you.