Pill
You are missing the point.
This is not a debate about 4 man v 3 man or even rest achieved. It's not even a debate about overall experience levels on the flight deck, or even the demographics of the pilot group, which will see an enlarged group of SOs waiting for upgrade to fewer FO positions.
The guts of the safety debate is we will invariably have someone sitting in the operating seat during the critical approach/landing phase who has not had the opportunity to sleep during anything like their optimal sleep window and they have probably encountered the exact same problem 48 hours earlier. In fact, they will probably have a career based on it!!!
Example, the JNB leaves around midnight HK time. The FO will envariably work first and will get a sleep opportunity around 7am body clock. He will then be back in the seat around 1300 body clock having not slept much. I would suggest that's not the real problem. The real problem comes the next day when he returns to HK. He will be expected to go from the bright sunshine of Africa and get 5 hours sleep and then sit in the seat (in the dark) all the way to the gate to HK. He will make the approach intomHK during his WOCL having had only one sleep opportunity during his normal sleep window in 4 days.
If Clockwork found the 3 man 256/254 unacceptable, then I cannot see how they will find this acceptable. That of course assumes they will ever be involved or see any data.
Call me a cynical old fading rockstar, but I wouid wager JNB and LGW were chosen because they are heavily requested and the company anticipates few complaints, until it all goes wrong one day.....