I've never heard of anyone who bought the Borla catback for the Impreza.
It's there...but I would stick to the more popular (and proven) exhausts such as the Sti, Stromung, Apexi, Tanabe, 5 Zigen, Scoobysport, etc. But that's just me, the performance differences are marginal for exhausts.
Also, I heard that in some magazine test like a year or two ago the Stromung had the best performance gains of a group of exhausts tested for the RS.