muffler swap vs new catback

[Sburn]

I got a very honest opinion from the guy who installed my Flowmaster:

Ford did a really good job on the exhaust system on the 2004+ F150s. He didn't think there was likely any HP gains to be had unless somebody went with a hi-flow cat, true-dual system, and then only a couple HP, at best.

He thought there was only two good reasons to change the stock set-up: 1) because you want it to sound better, and/or 2) to save some weight by dropping out the behemoth stock muffler.