Ah this video from BigX event. Those people are all well known D1 drivers, off course they are good. Not only Silvias, take a closer look you can see Skyline R32, Skyline R34, Soarer, 180SX...
Scenario 1: 2.5" from downpipe to straight flow muffler
Scenario 2: 2.5" from downpipe to straight centre box (also a silencer) to straight flow muffler
Scenario 3: 2.5" from downpipe to s-flow...