yes headers ARE worth it, and not just for sound, power ups too! they are just one more peice the the puzzle. cam, chip, intake and sense you already have great cat back. this thing should fly quite nicely. even on a 2.0 headers are worth the power, and sound.
well really trubo'ed cars only have down pipes in my book. yea our cars are in two parts the exhaust manifold and the "down pipe" but it wont do anything on our cars if you change that, and there isnt an upgrade for that "down pipe" just stock and headers.
is it constant shhh or a sshh sshh sshh? i have redone everything in my car with poly bushings i dont have any metal noise. the one noise that i do have is the diff humming because its on its way out lol.
did you check your pads? if a a slide got stuck on your caliper, then the pad would wear down to the metal that its attached to, and cause that sound. that has also happened to me. or it could be the pad is touching the rotor a little bit.