There is one thing for certain than I know.Owning both a V6 and a 5.0.You'll never get that V6 to sound like the 5.0.
The reason many V6'rs go the true dual setup,It's the only way to get a real sound change out of that exhaust.The exhaust on these cars sound more like a straight 6 than a V6.The Y pipe plays a big factor with this.A performance muffler wont give you much of a good sound.It's still too quiet,and sounds nearly the same.
Even if you gut out the cats,it may be a lot louder but it doesn't completely get rid of that straight 6 sound.There are however,2 methods of getting a real good sound but it will require extra work and time by you.I don't want to get too technical here so I'll stop.
I don't know what your gf is looking for in sound.Your best bet,go to mustangexhaust.com .It has TONS of sound clips.Some V6's even.