Veruka,
Check your maths. Pretty sure based on what I'm looking at that A350 would just barely make JFK with full pax and no cargo. 777-300ER does it with full pax and 10-15T cargo. That's not a full belly hold, but it's not as empty as the A350's would be. Haven't looked at any ETOPs issues as I'm sure they will be sorted in time.