I have a DP on my 06 and I am getting a LOT more noise from the passenger side, where the DP is located, then when it was stock.
I think what you are hearing is the turbo spooling and pushing air into the system. At what RPM do you really hear the sound? Mine is a bit more engine noise (read boxer ping) untill around 3k and then the "wind noise" starts. It's a somewhat high pitched whisstle sound that keeps building untill I let off the throttle and then I hear a slight expulsion of air. What all that is, is the turbo spooling and forcing more air into your system and then your BPV (by-pass-valve) letting go of the excess when you let your foot off the gass. It's quite normal from what I understand, especially considering how restrictive the stock DP is.
I would not mess with what Cobb have engineered. If you want more sound you should not have purchased a system that prides itself in being, "not much louder then stock". Prying off the muffler and adding another one will probably be as expensive as getting a new cat-back system from one of the great vendors we have access to through wrxTuners.