Sorry, I re-read my post and it may have sounded like a personal attack, which was not the case. For that I appologize (if that was how it was taken).
If subaru makes them and it is possible to do, go for it. It's your car and you should do what you want with it. I just know that I have heard them before and didn't care for the quality nor the "vibrating seat". Granted, what I heard was a 12" custom floor mounted subs (some of the floor pan was cut to make room for the sub).
As far as my understanding of the WRX, I may be incorrect for the newer models (newest generation). The WRX STI was a 2.0L Turbo Charged motor making roughly 300bhp (base horse power or crank horse power). The base model WRX was, as far as I remember, a 2.0L turbo charged motor that made roughly 200ph. I don't know much about the newest generation but I assume they would have kept the same trend.
NA= Naturally aspirated. In other words, the air is not forced into the motor via a supercharger or turbo charger.
FI = Forced Induction (IE Turbo of Supercharger).
If it is a spoiler that you want for the form of the car, go for it. Just remember that most require rigid mounting to the trunk or rear quarter panel via drilling holes and hardware.
I hope that clears a few things up. Good luck to you with whatever you choose. BTW, I like the new impreza's look much more then the old.
EDIT: The North American Subaru Impreza Owners Club (
http://forums.nasioc.com/forums)is a really good source for information, I suggest checking there.
-Jake