It has already been said that cat's all mature differently and maybe it is appropriate with some kittens / breeds and not others.

My vet has said to me it would do my kittens no harm or disadvantage to go to their new home (assuming the new owner vaccinates the kitten) at 8-9 weeks - he has also said that when all the kittens leave overnight this would be stressful for mum and it is better to stagger it over weeks.

I have decided to stick with the 13 week rule - not because i do not believe the vet but because a breeder gains a reputation and following advice laid out by the GCCF is obviously highly recommended - BUT that doesn't mean the vet is wrong.