Ended up flying QR836 in the 777-300. Despite getting the extra legroom of 39D, I would still give it a thumbs down.
Unlike 69D on the A380, 39D doesn't have a seat in front so no pocket to put stuff in. Also, there must be some difference with how the aisle narrows, because the FAs were constantly bumping me (and I wasn't stretched out at all). They were being very nice so I really think it was just misjudging how narrow the aisle gets so quickly.
Also, the USB port on 39D is on the inside of the seat, meaning that there's no room for the cord to sit naturally if anyone is in the seat, and the seat is too narrow to have something plugged in without it sticking into you. My phone charger was irreparably damaged by the end of the flight and I had to buy a new one at BKK. So overall, not a great seat for me. I don't know if the seats and aisles on this plane are more narrow than others, but it sure felt like it!
The flight was fully booked, which is never pleasant on a red-eye. Hopefully I'll be able to fly up front next time, but I'm not sure if it's possible to avoid this plane on this route in Y. If they are running it at full capacity often, I don't think I'll continue with my QR status plan.