Get a 2.25" cat back piping with a 24" Magnaflow muffler and keep more money in your wallet instead of shelling out a couple hundred more for the TRD on the exhaust tip.
The Magnaflow sounds nice and throaty but not obnoxiously loud,if you want that then go with the 18" muffler. Any HP gains would be in the mid range but it's not going to be real noticeable,it's more of a mod for sound and that mod is going on my truck by mid summer.