Did you just buy the oysters???
I think that I would have cooked them for less time too, as they probably don't need much cooking, although I have never cooked them on their own, they have only ever been on the bird. And I cannot remember the last time I cooked a whole turkey.
I was thinking that if I could get them here, I'd try breading them, that lovely soft meat with a crunchy outside, along with a similar sauce and veg, lovely.