I have flown on the 787 in J twice now within a week and I don't know, the seat is a little tight, sure, but I had no trouble sleeping, I'm not a big guy coming at 179cm and not very big in any other way neither.
i had enough space to place away my items, phone, passport and so on. When lowering the armrests i find it spacious enough to sleep comfortably. Both my flights were in the 10 hour category so proper long-haul. Will happily fly it again in the future. Only thing that annoys me is the "wing" of the seat blocking the window in A and K seats otherwise Iam a happy camper.