corycds

So i have a 2003 f 150 with the 5.4 and it needs to be loud. So i cut the muffler off and the sound just isnt what i want so i decided i want some highflow cats

Now i dont want to spend a fortune, so i want to replace just the rear cats (not all 4) with highflows. The guy at the muffler shop said it wouldnt do much good not replacing all of them.

So basically would replacing just the rear 2 cats with highflows make it sound louder or would it be the same?