Most catback systems are separated in 2-3 parts, and will have no issues fitting in your trunk if you take them out of the box. You fold down your seats for even more storage.
The stock catback won't fit unless you cut it down since it's all one piece.
I just got a Solo catback in mine. My stock catback was cut for Borla axle backs. Both the stock catback and axle backs fit in the car with rear seats down. The catback was touching my shoulder though.