I have almost this exact setup. I have a custom 2.5" DP that is identical to the KO, a custom 2.5" mid-pipe (with a flex in it) and the GReddy SP.
While I have not compared power on this setup with a 3" setup, I can tell you that going from stock to this setup added 2.5 PSI to my base boost (no controller) and made a very appreciable difference in performance and spool-up. I would think this mod alone is worth 20-25 hp on a stockish setup up to about 250WHP. I don't know how much more power a full 3" would make, but I wouldn't expect more than maybe 3-5 at the 250whp mark, and maybe 6-7 at 270-280 mark. I'm not sure if that justifies the cost of upgrading to a full 3", which would cost you about $1000 (TKO plus DP). I would think that $1000 would be better applied to a turbo or IC upgrade.
The sound with this setup is indeed louder than the OEM, but still reasonable, and it has a low rumble at idle. At WOT and at speeds over 90mph it is definately much louder than OEM, but not nearly as loud as any of the 3" systems I have been around, and not nearly as loud as PEX or ApexI GT (no silencer) either.