An exhaust system only flows as good as its weakest link, So what that means is, if you have a free flowing system that will lower your back pressure and still have the stock cats the system will only flow as much as the stock cat will flow even though in front and in back of the cat has been opened up. so make sure you off the cats and yes there is a big gain in power.
I dont have headers, but the magnaflow catback has a pretty bad drone at highway speeds. I find cracking the windows helps a lot to get rid of it. I wouldn't mind getting rid of the cats, but I remember how bad my mustang smelled without them...this would for sure be worse/
FWIW, I have RandomTek Exhaust with hi flow cats (emissions required here) and B&B headers. Added 20-30 HP. Mine is 439 RWHP now (490 RWTRQ). Guessing it was around 410-415 hp stock. These were done before I bought the car but I think the cost was around $2K. I think it was worth it as I love the sound and love to brag about the numbers.
Interesting, my dyno numbers were 439/490 bone stock..Wondering what I would see with the exhaust mods. Since then I have done K&N air filters, smooth tubes and cat deletes and SCT tuner with a cookie cutter tune. Wonder what I would see now?