I don't know if it's still true for the 2013 WRX but in my cars day (03) it was foolish to even think about getting a CAI for the WRX unless you were well past the 300HP mark.
Without a lot of other mods and/or a re-flash it adds zero HP. It may even hurt if it messes with your MAF censer.
In fact if you even recommended a CAI for a WRX, that was a good way to smoke out the people who didn't know what they were talking about.
Did I miss something that changed?
The sensation is as hard to forget as it is to ignore!!!