Planning on beefing up the WRX already? >
+1 on just get the best deal you can.
Short answer about the same.
The issue is the divorced setups can't separate the 2 exhaust flows effectively enough to show any major gains over the bellmouth setups.
Stay away from divorced setups that meet up early, bellmouths that have a sudden change in size, and catted ones where the cat is far up stream. The quality of the piece will make more difference than the style and even that will make almost no difference.