Are you saying they can't get airborne at 280t out of JNB? The 2% reduction in burn rate is roughly 3,000 lbs on that route. Was it really that close with a full load of fuel? The extra GW gives them 24,000 lbs of additional payload. This let's them carry cargo and passengers (revenue) in addition to fuel. Without the revenue from the extra weight, the range is pointless.