Headers and Y pipe give aprox 12hp gain. The less restrictive cat-back prob 5-8hp.
I know after all the work required in installing the headers and cat-back...it would be nice to hear 30-40hp gains...but its just not happening on these cars.
30-40hp = 3L swap.
As far as drone. You'll need to keep the cat...get a high flow one from Trubendz. And if you get the non-resonated cat-back you will have drone...a lot of it at highway speed. I have MSDS headers, Weapon R Y-pipe, Trubendz resnoated cat-back. Sounds really good...smooth and powerful. And no drone at highway speed. Not as good as my True Borla cat-back...but not as restrictive either ;-)