Yeah, headers help a little off-the-shelf with a little extra HP and fuel milage bump.
The big differences you can get out of a 454 without ripping apart the motor are LS1 28# injectors, headers, 0411, and a good tune. 3" out the headers to the muffler is good, but it needs to be necked down to 2.5" due to the HP you'll be making.
Once you get PE mode activated, played with, AND have tow-haul mode... you'll think you've got a totally different truck.
You're headed in the right direction, going with headers. It'll be worth having. It's been about 4-5yrs since I put my Gibson's on with he titanium ceramic coating. There's just a SMIDGE of surface rust, but you can't tell unless you really look close.
Engine masters in motor trend has a good video of muffler comparisons for horsepower. I like a nice rumble and snap of glass packs, but I could probably grab some HP with the proper mufflers. Oh well.