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.
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2007 F150 XL, 4.6, Regular Cab, Gryphon Installed 2/2009
"voiding warranties since 1979"
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