My guess is that it's entirely cosmetic. It does balance the rear, but strangley enough, Subaru choose to leave the muffler cans hanging low & very visible?...
The 2008-back muffler is 9.5" high and 10" wide and 17 inches long. It is a rounded box, almost as high as wide. The 2009-on muffler looks oval and shorter. I don't think there is enough room for the old box muffler to fit under one side of the new body.
Can 2009-on mufflers be hung any higher? If so, the tail pipes would have to emerge higher, requiring bigger cut outs in the bumper.
Does anyone know if its a true dual exhaust?
You can tell just by looking under the rear. The single pipe is split by a Wye to go to each muffler.
There are no true dual exhausts these days except perhaps on some muscle cars and muscle trucks. It requires two catalytic converters.
Rear drive V-engine and Boxer-engine vehicles lend themselves to true dual exhausts, where each bank of cylinders has its own path straight out. But very few such vehicles do.
True duals are possible on rear drive inline engines, but require a split manifold, like the old Pontiac OHC Six, and the present twin turbo BMW Six.
All the "dual exhausts" you see on new cars are "pseudo duals" because these cars are either inline 4 (like Miata) or they are transverse 4 (Nissan Altima) and these fours cannot split the manifold. Or they are transverse 6 (too many to name) or transverse 8 (Lincoln Continental) that must gather the front manifold into the rear manifold and then split the system after the converter.
All the dual-exhaust Subarus gather the left manifold into the right one. So they are all "pseudo duals", even though the boxer cylinder layout is perfect for true duals.