Yeah, thats about all it does, make your stock intake louder.
I havent seen any dyno graphs with just the silencer mod, but I dont think it would even give you 1 hp. A few people thought that they really tested the mod on a dyno and that it gave them a few hp atw, but it was not a true test because they had a K&N at the same time. Personally I would not want to spend the time to do it, just for a little more noise.
Well, if you do the silencer mod you should definately get a K&N at the same time. I did mine and I enjoy the extra sound. Now of course, I have a bunch more stuff done since then, but I can still hear the improvement in sound eveytime the turbo spools or the BOV releases pressure.
BTW: It's a very easy mod to do. Just make sure you have a long socket extension to remove the airbox.
I just did this mod this weekend. It does make things a helluva lot louder. I also installed the TCG intake kit which locates a shiny steel velocity stack inside the fender well directing cold air into theair box.
Lots louder and a minimal seat o' the pants difference. I'm happy tho.