Sure, but I thought the (Genex powered) 787 is FAA approved 330min ETOPs, so no different to the 777-8, just no QF approval as yet by CASA (which would be the same for the 777 anyhow...) Shouldn’t be to hard to get sorted with enough experience operating the type.
Im well aware Joburg is 5000 plus feet, that’s why I asked the question re; performance, quoting your typical summer weather for the departure times (at least when I’ve operated there)
Otherwise, I’m not sure why it wouldn’t be a good fit to replace the 747, reduced capacity notwithstanding.