It wouldn't give you gains if you don't have any other exhaust mods. The catback would just be more restrictive than the downpipe.
Thats like using a straw, but having one portion of the straw be bigger than the rest of the straw, its going to build up in there and you'll have to push more to keep the same velocity.
wut? the catback, even stock, isnt restrictive.
a 3" downpipe is good on any car, whether it is n/a or not. the fact is that a 2.5" downpipe is still a limiting factor, so just go 3" and be done with it.