Your Mom sounds pretty cool.
In MY 2008 the exhaust layout changed on turbo Imprezas, not sure about the FXT. Freeway drone and excessive exhaust noise probably isn't her thing so try to find an axleback that isn't super loud. I don't know if there would be a point of going with a CBE if everything else is staying stock. There's probably a resonator in the rear pipe that will help keep things quiet too, so I'd just go for an axleback.
At the same time, don't go too quiet. When we first put my Borla Hush CBE on, the rest of the exhaust was stock and it barely added any volume at all. It was only a touch more burbly. But now with the DP, HFC midpipe, and UP, it's perfect. Not too loud, but def. not stock.
Check out the FXT boards. I don't think any stock manifolds on turbo Subarus are EL from the factory.