It's neither an advantage or a disadvantage, from my understanding.
Your mark is just calculated the same way as it would be if you were in a group of people; just that your mark now constitutes that entire group.
Pretty unhelpful, I know, but if you're doing well in ancient and you like extension, why not give it a go? You can always drop out if you don't like it.
Otherwise, have a chat with your teacher and maybe your year coordinator.