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.
The STI exhaust is rather spendy for what you get.
Stromung, Borla, Brullen, Scoobysport are all good exhausts. A few people over on the other club have the Borla and seem to like them a lot. Even more people have bought the other three and love them too...
If you do get a Stromung, just remember to stop by Home Depot to get some High Temp. Grill Paint. It'll add some extra protection against rust in some of the more rust-prone areas of the exhuast. I'm using it with my Mild Steel, but I hear it may help on the SS versions too.
Besides that, great sound and butt-dyno gives it an A rating!