Bright Spark here who remembers the layout at Chicago O'Hare, up to about 15 years ago, with a 6-runway intersecting star pattern just like Heathrow once had, and the throughput they got from it was astounding (must be others here who recall it as well). And yes, they would have all six in use together. Dim recollection they got over 100 departures an hour. Like Heathrow, they eventually ran out of ground space as more the limiting factor, and have gone for substantial expansion.