If your girl is already good enough to handle her grade, why defer her? Beware that by deferring, you may bored her with a grade level below her ability and may have bad effects.
My experience - my younger sister (of course that is many years ago) was born in late Dec. My mum first send her to P1 in a quite good school & she was #2 in her class. Then my mum transferred her to a top school in HK to repeat P1. From then on, she was always just middle in class & never particularly good academically.