Arenberg (as they do it now) goes downhil 2-3% in the first 500 meters. After that, it undulates but very little and where they come off and go left is the lowest point of the secteur.Was it downhill just before Arenberg earlier or also slightly downhill along the cobbled section? If the there is uphill along the section going the other way, it should provide even bigger chance to create gaps in Arenberg (save from crashes).
The stressfull part is those first 4-500 meter where everybody arrives at 50+ K/hr and goes 2-3% downhill so no other option than to try and keep that speed while the bike/wheels feels like breaking up any moment.
It's a 3-3:30 minute effort at 400-450 watt.
If they would arrive at the opposite direction, the speed they enter will be slightly lower but not by much.