We took our F3 boy home at 9 weeks (always thought it was 8, but just reviewed the papers). Socially he was definitely ready to go, although his tummy wasn't really ready to take on any food that he wasn't eating at the breeder. At the breeder he was eating raw and kibble, but no canned food. Not that we didn't have intent to go to a raw diet, just weren't prepared to do it so quickly. When we initially ran out of the raw and we were making the transition to canned, he didn't tolerate it well with loose stools. Not wanting to feed him kibble, we got him back on a raw diet and everything has been good.
He does still take a while to transition to any new foods if switched abruptly, but probably not any different than any kitten. As much as we try to prevent it, he does love his kibble and we give him 20% of daily intake in kibble with a puzzle feeder. Also, if he decides he doesn't like a particular food, a little sprinkle of kibble over top is enough to get him to eat it.
Not any major issues just wish we would have been dialed in a bit better on the foods we were sent packing with. Might have taken a week or two longer to make sure they would tolerate what their new home was providing them. I also think it's important for the new owner to make sure they have a healthy supply of whatever the breeder was feeding their kittens so there aren't any abrupt diet changes.
Socially, I believe it depends on the kitten. Physically, I am in line with the 12 weeks that others recommend especially when a new kitten is introduced into a multi-cat household. Breeders saying yes to their clients isn't always a good thing, and an extra month of waiting given you should expect to have a long relationship with your kitten is certainly prudent.