As there are no long term studies I’ll throw up some anecdotal evidence. Brisbane based 747 crews for 3 years doing back to back JFK patterns. They were shattered, sleep patterns destroyed, sick leave up. A management pilot admitted to me that if the 747 wasnt finishing up they were going to close it down. One FO is still on long term sick leave. Also with both direct Heathrow and JFK rules have to be sorted out . Past experience has shown that you shouldn’t do both ports in the same roster for fatigue reasons . I may be laughed at but I’ll go on the record now as saying that there will be a lot of grief from this, when we had 30 747s I flew with an FO one day who had been on workers compensation for 6 months suffering sleep deprivation. Be very very careful what you sign up for.