the way i see it, just 1 more subject to study for.
when the real thing comes, your exams will be spaced out over 4-6 weeks depends.
i did arabic but my situation is miles away from yours but i did it as a private extra gcse but since its my home language its different to greek...
research ur schools exam board and see how the exam is set out, but if greek is where u want to get, u might as well take it