Fuji K hit it on the nose. This is why the rest of the world called the sump a 'twin scroll' sump and we here in the states call it an 'STi' sump.
Sloshing is not so as much of an issue as the boxer engine design
If you're at that level of performance you should be running an aftermarket setup anyway. Where there is a difference in the two OEM pans, is the baffling.
The older 'flat bottom' style pans do not flow oil down into the bottom of the pan nearly as well as the newer style. Oil will go every which way before making its way down and the large bottom gives the oil a lot more room to move around in, even with the baffle. The newer style has a much better flow through baffle design for letting oil make its way down quickly. On top of that the bottom is shaped like a tub to make sure oil in the bottom is always pooled around the pickup.
When Subaru changed to the 'twin scroll'/'STi' design they changed more than just clearance for factory EL manifold. Baffling and tub shaped bottom improved oiling performance. It's not uncommon to see starvation (on track) with the older pan design, but while still possible, more difficult with the newer design.
Along with the pan design, the pickup and baffle were updated as well. The pickup is less likely to funnel and the baffle is a sligtly better with oil control as well. All these aspects are why we recommend a (older EJ20) car that sees occasional track use to to consider switching to the newer 'STi' design, for improved oiling performance. Beyond occasional track use, a performance high capacity pan will provide and an even higher level of oil safety and performance.