I don't know for sure about you 2015 but I know on the 2011-2014 models you can buy the 'intermediate pipe' that my 2009 comes stock with and replace your resonator with it. It is just a straight piece of 2.5" with the OEM connections on it. Most guys do that and are happy with the sound. You can order the part direct from a Ford dealer or online parts place and it bolts right in place, no modifications required.
If you want/need a muffler swap I just recently put a GMPP Borla muffler on my 2009 5.4. Yes, it is a GMPP muffler on a Ford. The issue on these trucks is that Ford has a 3" Y pipe at the cats necked down to 2.5" at the intermediate pipe then it flares up to 2.75" on mine and 3.0" on yours. So, GMPP has a Borla muffler that is 2.75" and I picked it up for cheap directly from Borla. You have 3" so you will have MANY more options than I did for a muffler swap.
Just be careful, the 5.0 can get raspy in a hurry in the F150 if you uncork it too much.