Welcome, and just an fyi, Magnaflow has a DI/DO muffler with an x-pipe already inside it, so it might serve the same purpose and save you a few $$
, The only problem with exhausts in our trucks is that anything that is cat-back can only help so much... I dont know if the 4.6 is different, but the 5.4 has 4 catalytic converters, 2 per cylinder bank, and they are in series, so unless you get rid of one of those or replace both of them with some super high flow cats you will only get a limited amount of HP and mpg gains. but most people only do an exhaust for the sound anyways, so of course it is up to you. oh and btw removing a cat might be illegal in places, so I would check with an inspection guy or a cop or online to find out what the deal is.
hope I helped!