I believe the concensus on the more popular brands is that you can expect an 8-12 hp gain at the back with these catbax. A slight weight issue with SS vs the TI which was stock. Someone said earlier that the hp was almost incidental to the overriding desire for a different exhaust note.
BTW - as to all of this weight stuff - the DRIVER probably makes a much bigger impact than we acknowledge! Wonder during factory testing for ultimate numbers what "constant" driver weight was factored-in? 100# variation in driver weight will certainly impact 0-60 and 1/4 times when you are dealing in tenths or hundredths of a second!
So buy the catback for the sound or look and you might pick up a few ponies for good measure. :usa: